Ensure pre-roll frames are displayed when gstreamer backend is paused.

Perform a seek before transitioning from the stopped state to paused or
playing to force the pipeline to resupply the video sink with any
pre-roll buffer it may have previously ignored during loading.  And
don't assume showPrerollFrames to be true if the current state is not
stopped as the policy handling may have prevented an effectual state
change.

Change-Id: I288a70bc4da32f3534eab4b14702ca8f8fdb4222
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>