Convert locksettings unit tests to use FakeSettingsProvider

To mock out settings values in the locksettings unit tests, use
com.android.internal.util.test.FakeSettingsProvider instead of the
custom solution that was used before.

This makes mocked settings accessible to SyntheticPasswordManager, via
the Context using the standard methods.  The previous solution only made
mocked settings accessible to LockSettingsService itself, and only when
the special accessor methods in LockSettingsService.Injector were used.

Test: atest com.android.server.locksettings
Bug: 254378141
Change-Id: I16cfc1fc25cc55e66121fc9512fec01915de4885