Add android build script

[jg-hot]

Added an android subdirectory for building a prefab package and .aar with the Android NDK and gradle build system.

The build script supports building ready-to-use android library (.aar) via: cd android/ && gradle assembleRelease

Tests may be run on a connected device or emulator via: cd android/ && gradle ndkTest

See the included documentation in docs/android.md for details.