Cleanup of video pause button.

The "pause video" button had a few issues that needed to be cleaned up:
1. The accessibility text for the button was always "pause video".
I've changed this so that it says, "Turn on camera" or "Turn off camera"
to better indicate what pushing the button will do.
2. When the user accepts an incoming request to initiate a 1-way video
call, the button state would not be correct (the user's camera is
disabled, but the button would not reflect that).

Bug: 27944419
Bug: 28550284
Change-Id: I2594971ec21b5714ca2d750894a93b758f7a80e5