Add support for call handover from Telephony.

Add new config.xml value to indicate whether telephony supports handover
of calls from it to another ConnectionService.
Add logic in TelephonyRegistry to set PhoneAccount extra to indicate
when handover is allowed.

Test: Manual
Bug: 37102939
Change-Id: If5b7deb6b73342213b0158a82f1900eb7c962ffe
2 files changed