Upgrade / downgrade requests

- Remove unnecessary "change to voice" button.
- Remove "modify call option" dialog that showed up in the past when
pressing the "upgrade to video button".
- Cleaned up some unnecessary Toasts for call session events, data
usage, and video quality
- Implemented the camera on/off functionality.
- Change preview to show the user's profile photo when their camera is
turned off, as well as a "camera off" icon.

Bug: 20257400
Change-Id: I6b7bb3d2394467f1345a798557130b2a674337c4