[Android] Destroy extension when activity is destroyed

This fix is to ensure that XWalkExtension object gets destroyed when the
Activity is destroyed (e.g. press back button).

Since each XWalkExtension object on Java side has a corresponding native
extension object, we need to make sure the native extension object is
also destroyed. Otherwise, the leaked native object will cause unpredictable
program behavior.

BUG=https://github.com/crosswalk-project/crosswalk/issues/1023