Add support for disconnecting outgoing calls for emergency calls

1) Add CarrierConfig to allow/disallow holding calls for emergency
calls.
2) If holding an ongoing call for an emergency call is disallowed,
define a new DisconnectCause reason to notify telecom/dialer of this
disconnect reason.

Bug: 138741228
Test: atest CtsTelecomTestCases; atest CtsTelephonyTestCases
Change-Id: I7c6a1d0f648c598111ed047fbb9c609843c6aba9