Persist the nit brightness for the default display

Store the brightness for the default display in nits in persistent data store so that it can be persisted if the device reboots or the display device changes.

Bug: 258455654
Test: adb shell dumpsys display | grep -A30 PersistentDataStore
Test: adb pull /data/system/display-manager-state.xml
Test: atest com.android.server.display
Test: adb shell dumpsys display | grep mPersistBrightnessNitsForDefaultDisplay
Change-Id: I20a0a686ee1e4e945fbcd99d5b996534f8ef93b7