Add accessibility appearance related B&R function for Onboarding

Add entrance for B&R accessibility appearance related settings.

Flag: com.android.settings.flags.accessibility_appearance_settings_backup_enabled
Bug: 349941894
Bug: 328585994
Test: manual, need to open aconfig flag.
Change-Id: I9617da947201def01f2cfd7f20ce3d11d65f882d