Change the method of obtaining IMS registration radio tech and add test case

PhoneSwitcher tries to register a callback to obtain IMS registration
radio tech. But this callback will be removed when the binding with
ImsService is changed. This is a timing issue caused by operating in a separate thread.
To avoid issue, using ImsPhone#registerForImsRegistrationChanges() to receive IMS registration radio tech instead of registring a callback into ImsManager.

Bug: 330120237
Bug: 333300524
Test: atest PhoneSwitcherTest
Test: manual test and verify based on text log
      after IMS registered -> wifi on -> APM on -> APM off -> wifi off
Change-Id: Ia1220faea739bce58e1c54889341e8dba899302b