stagefright: explicitly mark discarded buffers

Return buffers always, but mark discarded buffers explicitly.

Bug: 33059528
Test: (manual) open a website with a looping video and see that it works.
Test: adb shell am instrument -e size small -w 'android.media.cts/android.support.test.runner.AndroidJUnitRunner'
Change-Id: Icbe12d09b5721747437592e11def2302f4c8bf79