Slim: Build stagefright ffmpeg plugin on BOARD_USES_QCOM_HARDWARE

Revert "Revert "slim: Build stagefright ffmpeg plugin""

This reverts commit 7130751405ee48ab8b8746a350c42a10746a217f.

Change-Id: I373ef0a5668eb3beb66698321b5253ea469b4b5f