Auto downgrade emergency video call to audio-only if not supported.

Based on the new CAPABILITY_EMERGENCY_VIDEO_CALLING PhoneAccount
capability, auto-downgrade to audio if emergency video calling is
not supported.  We do this early so that when the call is added to the
Incall UI, it does not waste time setting up video surfaces that will never
be used.

Bug: 27484017
Change-Id: I302070d885575e5ea0384803fdade39056f57c3a