Improved NDK r10 support for the Introjucer

Hi Julian,

This pull request focus on improved NDK r10 support for the Introjucer (so please ignore any OpenGL related changes :-P)

The following enhancements are added:
 * The APP_ABI in Application.mk can now be set separately in Debug and Release config.
 * The NDK_TOOLCHAIN_VERSION in Application.mk can be set (default: 4.8), so that users can use clang as an alternate toolchain.
 * Fixed some errors and warnings for the the clang toolchain, includes:
   * "LOCAL_ARM_MODE := arm" in Android.mk (it will causes errors when APP_ABI is set to "x86" or "armeabi").
   * "-Wno-psabi" cpp flag in Application.mk (it is ignored in clang and it generates warnings).
 * Fixed a Android build error when no application icon was set in the Introjucer.
 * Changed to use $(NDK_DEBUG) for distinguishing Debug / Release build.  
 
Cheers,
Ming