Introduce ANSWER_PHONE_CALLS permission

This exposes an API to answer a ringing call, as well as a corresponding
runtime permission and appop

Test: Grant the permission and ensure the call gets answered.
Deny the permission, and ensure that the API call throws an exception.
Bug: 30932767
Change-Id: I4c33fcea6b95a30469fa6c0c37090be32b0ad52e