Add new method to get the main Android activity.

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>