Introduce UT for TelecomAccountRegistry

To make TelecomAccountRegistry testable, the CL has to introduce minimum change below:
- Update Handlers in PstnIncomingCallNotifier and
  PstnPhoneCapabilitiesNotifier with Phone's looper. This is enforced
  when registerForNotifications is called. This is UT-friendly for the
  mocked Phone which can't specify internal looper.

This is UT test improvement only CL, no actual logic is made. So no flag is used.

Bug: 319648880
Test: atest TelecomAccountRegistryTest
Flag: EXEMPT test only
Change-Id: I2210a5d47e5b0970ec16513d5a4382c43861c7b8
3 files changed