There were no public APIs the user could use to get access to
their applications main activity, and thus far the only solution have
been to access undocumented and private APIs.

[ChangeLog][QtAndroidExtras] Added functionality to get the Android main
activity.

Change-Id: Ifd4121e8bc643360ed7a7038ffae9e872b59db71
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

The QJNI API's provide an easy way to communicate with Java API's from
C++. The API consists of two classes, QJNIObject which acts as a wrapper
around Java classes and QJNIEnvironment which attaches the current
thread to the Java VM and exposes the Java Native Interface.

Change-Id: I9cdc395249de74f940b75eab9f2bd32347d5d432
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>