libstagefright: Add 2 APIs (suspend and resume) in Mediaplayer

- API:suspend() will just pause the player and release all the decoders
  instead of release the whole player
- API:resume() will just init the decoders again
- Add a check in onVideoEvent()
  to make sure the first seek operation will seek the next i-frame

Signed-off-by: jmeng <jmeng@codeaurora.org>

Change-Id: If16dc1c9dae2f28df8a969b1044ca67d23a8846b