Add backing implementation for TelecomManger#isInSelfManagedCall.

Add backing impl for new method used by Notification Manager and
associated tests.

Test: Manual test with Telecom test app to validate method behavior.
Test: Added new unit test to exercise backing functionality for new method.
Bug: 226959797
Change-Id: I417a2c909c7db13d17b376c70004baa24281749b