[Android] Add target to generate AAR library

use command "ninja -C out/Release xwalk_core_library_aar" can generate AAR library,
the construct of this library is Root/jni/so Root/res Root/classes.jar Root/R.txt
Root/AndroidManifest.xml
the AAR library test successfully by gradle with different architecture.

BUG=XWALK-1930
Merge pull request #2270 from fujunwei/aar-library

[Android] Add target to generate AAR library