Create its own Handler instance for persisting display brightness

The BackgroundThread.sHandler is shared among the system_server,
removing the messages from it would break other part of the system.
Create its own handler instance when saving brightness into persistent
data store.

Bug: 235173102
Test: atest PersistentDataStoreTest
Test: dumpsys activity -a && check the content providers
Change-Id: I6345ec84ef7d01301234541e4415cafb0ad2075b