Add interface for radio service user controller

Refactored radio service user controller to interface to avoid
mocking static methods in unit tests, which causes test flakiness.

Bug: 363965571
Flag: EXEMPT refactor
Test: atest BroadcastRadioTests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:7f30249a02c059800bb4eb4b71fd8b22ffe2ee2f)
Merged-In: I1c05ff684bc969c704cb1531ee1c7efb84465003
Change-Id: I1c05ff684bc969c704cb1531ee1c7efb84465003