Re #1768: Fixed problems with Java garbage collection on call and buddy object

git-svn-id: http://svn.pjsip.org/repos/pjproject@4840 74dad513-b988-da41-8d7b-12977e46ad98
Re #1768: fixed wrong JNI compilation flags used by pjsua2 Android sample app

git-svn-id: http://svn.pjsip.org/repos/pjproject@4841 74dad513-b988-da41-8d7b-12977e46ad98
Re #1768: added APP_PLATFORM to avoid this warning "WARNING: APP_PLATFORM android-18 is larger than android:minSdkVersion 11" which would make native debugging fail

git-svn-id: http://svn.pjsip.org/repos/pjproject@4842 74dad513-b988-da41-8d7b-12977e46ad98
Re #1768: avoid crash if outgoing call fails during media initialization, because on_call_state() is called with NULL event, which is unexpected by pjsua2 (and possibly other unsuspecting apps)

git-svn-id: http://svn.pjsip.org/repos/pjproject@4843 74dad513-b988-da41-8d7b-12977e46ad98