Further cleanup of SecureSettingsRepository & friends

* Add a boolSetting() method to Secure/SystemSettingsRepository so that they can be used as a drop-in replacement in places that today inject the UserAware variants.
* Add unit tests for both concrete implementations of UserAwareSettingsRepository.

Bug: 356099784
Test: atest UserAwareSystemSettingsRepositoryTest
Flag: com.android.systemui.user_aware_settings_repositories
Change-Id: I904853ddac85b4040730560fb88de92dbed8f14d