Add new carrier config settings.

1. Add new carrier config setting for "bool_allow_emergency_video_calls".
This allows us to control on a per-carrier basis whether emergency
video calls are allowed.
2. Add carrier config setting "bool_allow_video_pause", which
was previously in overlay files in packages/services/Telephony (Figured I
would clean this up while I'm here).

Bug: 20089489
Change-Id: I97e0983b4a7c8e8086ab58e7e839011e62fabcaf