Modificated some Timer methods, implemented modifications in TimerTest, updated android target from 19 to 21

*[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>
In my case it was, but it wasn't obligatory in common. I am just working with latest api (21), and thought that it would be helpful to those with fresh install to work on source without downloading api-19 also.