android: update toolchain file to support ndk 10c

Toolchain update from https://github.com/taka-no-me/android-cmake/pull/23
android: use target SDK version for appcompat

Also Use ANDROID_APP_TARGET_SDK for application compatibility project.
It is now automatically set/updated during build.

Note: The ANDROID_APP_TARGET_SDK version need to be larger or equal to the
version required by the used Android support library (appcompat).
android: update documentation

Add Android support library to requirements.
android: update target sdk to 21

Use latest SDK platform (android-21) per default.