Formalize TelecomManager#isInSelfManagedCall

Make TelecomManager#isInSelfManagedCall a proper system API as per
mainline prep. Note that this is only used by the system UI.

Bug: 322516073
Bug: 311773409
Test: atest SelfManagedConnectionServiceTest
Change-Id: I364eab0aab66290dd96e5f23ee4dc52febaba384
5 files changed