Call handover improvements

- Set handover_from attribute when initiating a handover to another phone
account.  This lets the recipient know where the call is coming from.
- Replace "handover successful" indicator on calls with a handover state.
This ensures there is sufficient state to ensure that if a call is
hung up before the handover is accepted that we don't treat that hangup
as if the handover was successful.
- Ensure failed handovers are not logged.
- Prevent EXTRA_IS_HANDOVER from being set for outgoing calls.

Test: Unit, Manual
Bug: 37977155
Bug: 38027075
Change-Id: Ie5240c28163714be71825e718daa560eb568330f