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>
Add API to get the Android API level

The Android SDK version, or API level, is commonly used to determine
what APIs are available at runtime.

[ChangeLog][QtAndroidExtras] Added convenience method for getting the
Android SDK version.

Change-Id: Ic20322f68bc9680aec9e1c8404583462063904db
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>