Merge #17227: Qt: Add Android packaging support

246774e26459cb3652e308880abdd140e8e9d204 depends: fix Qt precompiled headers bug (Igor Cota)
8e7ad4146d55f472e3d1dacaabb6b7dee704a896 depends: disable Qt Vulkan support on Android (Igor Cota)
ba46adaa1abd51798394b5bad3799021adc237d2 CI: add Android APK build to cirrus (Igor Cota)
7563720e30a3052b7ee390f1b3d2874856fd073a CI: add Android APK build script (Igor Cota)
ebfb10cb75adb704418d08197681c1e742e63bd5 Qt: add Android packaging support (Igor Cota)

Pull request description:

  ![bitcoin-qt](https://user-images.githubusercontent.com/762502/67396157-62f3d000-f5a7-11e9-8a6f-9425823fcd6c.gif)
  This PR is the third and final piece of the basic Android support puzzle - it depends on https://github.com/bitcoin/bitcoin/pull/16110 and is related to https://github.com/bitcoin/bitcoin/pull/16883. It introduces an `android` directory under `qt` and a simple way to build an Android package of `bitcoin-qt`:

  1. Build depends for Android as described in the [README](https://github.com/bitcoin/bitcoin/blob/master/depends/README.md)
  2. Configure with one of the resulting prefixes
  3. Run `make && make apk` in `src/qt`

  The resulting APK files will be in `android/build/outputs/apk`. You can install them manually or with [adb](https://developer.android.com/studio/command-line/adb). One can also open the `android` directory in Android Studio for that integrated development and debugging experience. `BitcoinQtActivity` is your starting point.

  Under the hood makefile `apk` target:

  1. Renames the `bitcoin-qt` binary to `libbitcoin-qt.so` and copies it over to a folder under `android/libs` depending on which prefix and corresponding [ABI](https://developer.android.com/ndk/guides/abis.html#sa) `bitcoin-qt` was built for
  2. Takes `libc++_shared.so` from the Android NDK and puts in the same place. It [must be included](https://developer.android.com/ndk/guides/cpp-support) in the APK
  3. Extracts Qt for Android Java support files from the `qtbase` archive in `depends/sources` to `android/src`

  There is also just a tiny bit of `ifdef`'d code to make the Qt Widgets menus usable. It's not pretty but it works and is a stepping stone towards https://github.com/bitcoin/bitcoin/pull/16883.

ACKs for top commit:
  MarcoFalke:
    cr ACK 246774e264
  laanwj:
    Code review ACK 246774e26459cb3652e308880abdd140e8e9d204

Tree-SHA512: ba30a746576a167545223c35a51ae60bb0838818779fc152c210f5af1413961b2a6ab6af520ff92cbc8dcd5dcb663e81ca960f021218430c1f76397ed4cead6c

2 files changed, 13 insertions, 0 deletions

diff --git a/doc/README.md b/doc/README.md
index 19d8204d83..f32600d009 100644
--- a/doc/README.md
+++ b/doc/README.md
@@ -44,6 +44,7 @@ The following are developer notes on how to build Bitcoin Core on your native pl
 - [FreeBSD Build Notes](build-freebsd.md)
 - [OpenBSD Build Notes](build-openbsd.md)
 - [NetBSD Build Notes](build-netbsd.md)
+- [Android Build Notes](build-android.md)
 - [Gitian Building Guide (External Link)](https://github.com/bitcoin-core/docs/blob/master/gitian-building.md)
 
 Development
diff --git a/doc/build-android.md b/doc/build-android.md
new file mode 100644
index 0000000000..7a8a9e6a65
--- /dev/null
+++ b/doc/build-android.md
@@ -0,0 +1,12 @@
+ANDROID BUILD NOTES
+======================
+
+This guide describes how to build and package the `bitcoin-qt` GUI for Android on Linux and macOS.
+
+## Preparation
+
+You will need to get the Android NDK and build dependencies for Android as described in [depends/README.md](../depends/README.md).
+
+## Building and packaging
+
+After the depends are built configure with one of the resulting prefixes and run `make && make apk` in `src/qt`.
\ No newline at end of file