Improved cmake build script for Android:
- Error out when Ant or android can't be found
- Attempt to find the NDK toolchain binaries on Windows and Linux instead of assuming Linux
- Use find_program instead of find_path to find some binaries
- ANDROID_SDK now refers to the Android SDK base directory
- Fixed detection of android.bat on Windows
- Output of the android tool is now shown so that success/failure is visible
