write the diff number into Settings and take care of read_only

This change include

1. It will write the number of the differences between new storage and
   settings into the settings, so it will be easier for people to check
   if there is difference for each boot.

2. Read_only flags will not be sync in the bulk sync, and their server
   and local value won't be compared.

Test: atest
SettingsProviderTest:com.android.providers.settings.SettingsStateTest
Bug: 334947472
Change-Id: I5ed4cecd7488a56ad9e73d1f4c56bd76945fc871