Various Android fixes related to garbage collector and other crashes

[bennylp]

Fixes in sample Android application and also other places related to assertion and crashes, due to garbage collection issue (see ​GC issues in PJSUA2 book) or other issues.

[bennylp]

In 4840:

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

[bennylp]

In 4841:

Re #1768: fixed wrong JNI compilation flags used by pjsua2 Android sample app

[bennylp]

In 4842:

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

[bennylp]

In 4843:

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)

[bennylp]

In 4844:

Re #1768: fixed potential GC problem when rejecting incoming call

[ming]

In 4866:

Re #1768: move access to MainActivity? class from MyApp? to avoid undefined symbol error when not compiling for Android app

[nanang]

In 4906:

Re #1768: Add forced GC to libDestroy() in SWIG Java.