Properly support multi-user in NotificationSettingsRepository

(And also in every place that injected SecureSettingsRepository / SystemSettingsRepository).

This also:
* adds some methods to UserAwareSecureSettingsRepository (via the base class) and their tests.
* removes the UserAwareSecureSettingsRepository interface/impl/fake split.
* adds a UserAwareSystemSettingsRepository (same base class, but interacting with SystemSettings instead of SecureSettings).

Bug: 356099784
Test: manual, as described in b/356099784
Flag: com.android.systemui.user_aware_settings_repositories
Change-Id: Ic8d759de9cc93e59ae279ecce0c818b1f3d200e3