Addressing SecureLockDeviceService feedback / cleanup

Implementing feedback on ag/32553393: creating subclasses for storing
settings state / restoring on secure lock device disabled, adding unit
tests for serialization/deserialization. Also general cleanup, checks on
concurrency, etc

Flag: android.security.secure_lock_device
Bug: 401645997
Fixes: 396680098
Fixes: 440146591
Test: (manual) Ensure security features are active
Test: atest SecureLockDeviceServiceTest
Test: atest UsbSettingControllerTest
Test: atest SystemSettingControllerTest
Test: atest SecureSettingControllerTest
Test: atest DisableFlagsControllerTest
Test: atest DevicePolicyRestrictionsControllerTest
Change-Id: I02a06fd92453fac2f3b2273c74cb28f9818284c0