Flag for caching UserProperties correctly

UserProperties are currently not cached correctly. This needs to be
fixed, and callers need to optimize their calls to use the new caching
mechanism properly.

Bug: 369198539
Flag: android.multiuser.cache_user_properties_correctly_read_only
Test: N/A
Change-Id: I093855f26623bf32444f9741ec24c0d462ccfe4b