Android Support: Exporting to Android Archive (AAR) (#10271)

* added android triplets

* added android support to vcpkg

* added export directories to git ignore

* fix libraries naming

* added vckpg sources to visual studio project files

* rename file location

* issue with std::string fs:path copy initialization

* format path on VStudio

* fix checks format cannot work on fs::path

* support header only libraries

* support using architecture instead of triplets

* added prefab support

* added debug logs and prefab debug flag

* added support for empty packages i.e openssl

1 files changed, 124 insertions, 0 deletions

diff --git a/docs/specifications/prefab.md b/docs/specifications/prefab.md
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+## Exporting to Android Archives (AAR files)
+
+Vcpkg current supports exporting to android archive files([AAR files](https://developer.android.com/studio/projects/android-library)). Once the archive is created it can imported in Android Studio as a native dependent.  The archive is automatically consumed using [android studio's prefab tool](https://github.com/google/prefab). For more information on Prefab checkout the following article ["Native Dependencies in Android Studio 4.0"](https://android-developers.googleblog.com/2020/02/native-dependencies-in-android-studio-40.html) and the documentation on how to use prefab on [https://google.github.io/prefab/](https://google.github.io/prefab).
+
+#### To support export to android the following tools should be available;
+
+- `maven <optional>`
+- `ndk <required>`
+- `7zip <required on windows>` or `zip <required on linux>`
+
+**Android triplets that support the following architectures arm64-v8a, armeabi-v7a, x86_64 x86 must be present**
+
+#### An example of a triplet configuration targeting android would be
+
+```cmake
+set(VCPKG_TARGET_ARCHITECTURE arm64)
+set(VCPKG_CRT_LINKAGE dynamic)
+set(VCPKG_LIBRARY_LINKAGE dynamic)
+set(VCPKG_CMAKE_SYSTEM_NAME Android)
+```
+
+The following table outlines the mapping from vcpkg architectures to android architectures
+
+|vcpkg architecture | android architecture |
+|-------------------|----------------------|
+|arm64              | arm64-v8a            |
+|arm                | armeabi-v7a          |
+|x64                | x86_64               |
+|x86                | x86                  |
+
+**Please note the four architectures are required. If any is missing the export will fail**
+**To export the following environment `ANDROID_NDK_HOME` variable is required for exporting**
+
+#### Example exporting [jsoncpp]
+The `--prefab-maven` flag is option. Only call it when you have maven installed
+```
+./vcpkg export --triplet x64-android jsoncpp --prefab --prefab-maven
+```
+
+```
+The following packages are already built and will be exported:
+    jsoncpp:x86-android
+Exporting package jsoncpp...
+[INFO] Scanning for projects...
+[INFO]
+[INFO] ------------------< org.apache.maven:standalone-pom >-------------------
+[INFO] Building Maven Stub Project (No POM) 1
+[INFO] --------------------------------[ pom ]---------------------------------
+[INFO]
+[INFO] --- maven-install-plugin:2.4:install-file (default-cli) @ standalone-pom ---
+[INFO] Installing<root>/prefab/jsoncpp/jsoncpp-1.9.2.aar to /.m2/repository/com/vcpkg/ndk/support/jsoncpp/1.9.2/jsoncpp-1.9.2.aar
+[INFO] Installing <vcpkg_root>/prefab/jsoncpp/pom.xml to /.m2/repository/com/vcpkg/ndk/support/jsoncpp/1.9.2/jsoncpp-1.9.2.pom
+[INFO] ------------------------------------------------------------------------
+[INFO] BUILD SUCCESS
+[INFO] ------------------------------------------------------------------------
+[INFO] Total time:  0.301 s
+[INFO] Finished at: 2020-03-01T10:18:15Z
+[INFO] ------------------------------------------------------------------------
+In app/build.gradle
+
+    com.vcpkg.ndk.support:jsoncpp:1.9.2
+
+And cmake flags
+
+    externalNativeBuild {
+                cmake {
+                    arguments '-DANDROID_STL=c++_shared'
+                    cppFlags "-std=c++17"
+                }
+            }
+
+In gradle.properties
+
+    android.enablePrefab=true
+    android.enableParallelJsonGen=false
+    android.prefabVersion=${prefab.version}
+
+Successfuly exported jsoncpp. Checkout <vcpkg_root>/prefab/jsoncpp/aar
+```
+
+#### The output directory after export
+```
+prefab
+└── jsoncpp
+    ├── aar
+    │   ├── AndroidManifest.xml
+    │   ├── META-INF
+    │   │   └── LICENCE
+    │   └── prefab
+    │       ├── modules
+    │       │   └── jsoncpp
+    │       │       ├── include
+    │       │       │   └── json
+    │       │       │       ├── allocator.h
+    │       │       │       ├── assertions.h
+    │       │       │       ├── autolink.h
+    │       │       │       ├── config.h
+    │       │       │       ├── forwards.h
+    │       │       │       ├── json.h
+    │       │       │       ├── json_features.h
+    │       │       │       ├── reader.h
+    │       │       │       ├── value.h
+    │       │       │       ├── version.h
+    │       │       │       └── writer.h
+    │       │       ├── libs
+    │       │       │   ├── android.arm64-v8a
+    │       │       │   │   ├── abi.json
+    │       │       │   │   └── libjsoncpp.so
+    │       │       │   ├── android.armeabi-v7a
+    │       │       │   │   ├── abi.json
+    │       │       │   │   └── libjsoncpp.so
+    │       │       │   ├── android.x86
+    │       │       │   │   ├── abi.json
+    │       │       │   │   └── libjsoncpp.so
+    │       │       │   └── android.x86_64
+    │       │       │       ├── abi.json
+    │       │       │       └── libjsoncpp.so
+    │       │       └── module.json
+    │       └── prefab.json
+    ├── jsoncpp-1.9.2.aar
+    └── pom.xml
+
+13 directories, 25 files
+```