Cleanup of Video Call pause functionality.

Whether the paused videoState is available is dependent on the carrier's
implementation of the VT spec. The original VT implementation assumed
that this was stored in a system property accessed via InCall; these CLs
move this to a Call/Connection capability which will ultimately support
multisim video capable devices.

- Made some general cleanups in VideoPauseController (comments, some minor
logic changes, renamed a few methods).
- Removed access of system property to see if pause is supported and
instead key in on the call capability.
- Changed video call presenter to hide incoming video surface when a pause
signal is received.

Bug: 16680364
Bug: 19820114
Change-Id: I6d1a7789925c69e4022846f8d847b8bb1274c06b