*[gdx/src/com/badlogic/gdx/utils/Timer.java] Changed types of "postTask"...

..., "scheduleTask" and also static "post", "schedule" methods from void to Task. Now they are returning processed task in case of assignment to a field on call.

*[tests/gdx-tests-android/project.properties] Updated target=android-19 to target=android=21
*[tests/gdx-tests/src/com/badlogic/gdx/tests/TimerTest.java] Implemented changes in Timer class to TimerTest class.

Signed-off-by: Burak Kurkcu <burakkurkcu@gmail.com>