libstagefright: Signal input EOS after last buffer is released

-In offload playback, sink stop is called at EOS from the sink call
 back when input EOS is reached. This is called before the last
 buffer is released.
-During seek to the end of clip ,drain can complete even before
 last buffer is released. This can cause playback to switch to
 next stream.
-Fix by ensuring sink stop is called only after the last buffer is
 released.

CRs-Fixed: 607104
Change-Id: I9cc45cc25b5f87dd878b26f76ea741323a0242a6