Make android.telecom.action.POST_CALL a protected broadcast.

Ultimately we only want Telecom to be able to broadcast this and use it
to open an activity in an app implementing the CallScreeningService API.

Test: Added CTS permissions verifier test to confirm this is protected.
Bug: 169834488
Change-Id: I0ca1b680b6bacc2597a59714b2c1a3363fe8057f