Refactor BackgroundUserSoundNotifier

Remove unnecessary Context parameter from some methods where it can be accessed in another way.
Add notificationClientUid parameter to some methods to prepare them for being able to handle specific AudioFocusInfo once multiple AudioFocus handling is implemented.

Bug: 367615180
Test: atest BackgroundUserSoundNotifierTest
Flag: EXEMPT trivial refactoring
Change-Id: Ica949f278cc1e56901345413cfa2f5ccfefef20e