Gallery2: Seek to trim start time only once when playing

- When playing video in TrimVideo activity, it may enter an endless
  loop: Seek to trim start time (actually sought to previous sync frame)
  -> Periodic check, if current position < trim startm, seek again.

- This patch adds a flag to ensure seeking to trim start time only once.

Change-Id: I3cd74fbd85546cc35d5c32797ab7f1775f091586
CRs-Fixed: 694431