Allow user Java/C++ interaction

This removes everything in src/android/cpp. qtmain_android.cpp is no more
and is now integrated into src/plugins/platforms/android/src/androidjnimain.cpp.
It seemed fitting to integrate startQtApp() into the same file as
pauseQtApp() and resumeQtApp(). (Additionally this removes redundant copied
code, and simplifies how necessitas projects work and are built.)

This doesn't support using JNI with bundled libs. We have to be careful to
load certain bundled libs (maybe only the plugin .so) in the delegate class loader,
so I could break certain apps by making such a switch. Need a way to categorize
the type of bundled lib, in order to decide which class loader to use.

Fix https://sourceforge.net/p/necessitas/tickets/140/

Many thanks to Tyler Mandry <tmandry@gmail.com> for this patch.