Add CAPABILITY_SUPPORTS_VIDEO_CALLING to PhoneAccount.

Adding companion PhoneAccount capability which is used to indicate when a
PhoneAccount supports video calling.  That is, whether it can potentially
make video calls, but not necessarily at the current time.

This is an often requested OEM enhancement which is used to drive UX (e.g.
imagine a video calling icon showing up if the device supports video, but
only being enabled when the device is in range of a VT capable tower).

See bug for reference to design doc.

Bug: 27328615
Change-Id: I08fc18950e6d35a8a7df47ce37aa2326624b9fd3