IMS-VT: Set video call provider on call update.

VideoCallProvider is set upon creation of ImsPhoneConnection.
If creation of video provider is delayed in vendor code, e.g.
because UE is not video capable at that moment, then video
features will be disabled for the lifetime of that session.
This change checks and sets the video provider if it's not already
set upon receiving an update from vendor code.

BUG=25696226
Change-Id: I33bfde75726f64ead87f0b1e4b4005e4ddd90619