MoviePlayer: fix streaming video stepping forward

The default forward/rewind step is 3/6 seconds, which falls short to
reach another intra-frame. Change the default to 10 seconds.
Also, some code clean-up is made.

CRs-Fixed: 613577

Change-Id: I94bceb5cdac0904545ef58609855c8fb8d6aa05a
MoviePlayer: prevent black screen on resuming

If a user leaves streaming video and then returns to it, a black
screen is shown because of buffering.  Show a spinner to prevent this.

CRs-Fixed: 613579

Change-Id: Iceb658b3280aacfce3c82b9268b922a8d4d69333