DeviceStateRotationLockSettingsManager: only persist setting when value changed

This is to prevent the following infinite loop:
1 - We register a content observer for changes in our setting
2 - We initialize by reading the setting
    -> setting is empty
    -> fallback on defaults
    -> default is empty
    -> persist empty setting
3 - We get notified about the setting change
    -> back to step 2. Loops forever

Bug: 219652963
Test: Manually + atp unbundled/launcher/launcher3_test_platform
Change-Id: I08788d5ef7739c2f840a4f74459c4462c9db9c10