Add feature flag to change the method of obtaining IMS registration radio tech

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, ImsPhone provides IMS registration radio tech to PhoneSwitcher instead of registring a callback.

Bug: 330120237
Bug: 333300524
Test: build pass
Change-Id: I064cc66263d25741077a425e6589e490023409c7