Qt: add Android packaging support

Introduce an android directory under qt and allow one to package bitcoin-qt for Android by running make apk.
Add bitcoin-qt Android build instructions.

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

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`.