libstagefright: Fix for crash during lpa playback

-During lpa playback, flush() is not issued if mReachedOutputEOS
is true. This prevents flush reaching to HAL even during pause
state and causes crash.
-Issue flush() in all cases as these checks are handled in HAL.

Change-Id: I0029c851ba6a029afc01e6a30620b071f9828665