Support notifying Telecom when a call is ended due to new emergency call

When there is a new outgoing emergency call, the telephony
ConnectionService may choose to end existing calls based on
carrier configurations. This change introduces a new DisconnectCause
reason, which allws Telecom to pop up a notification when calls
in this fasion.

Also restricts the possible PhoneAccounts that can have simultaneous
non-emergency+emergency calls to those with the
CAPABILITY_PLACE_EMERGENCY_CALLS capability to make sure 3rd party
PhoneAccounts do not perform badly and affect the outgoing emergency
call.

Bug: 138741228
Test: atest TelecomUnitTests; atest CtsTelecomTestCases
Change-Id: I1e1836367080e8235ebb4818a49f2aa26ad1d7a0