[backup] system_server should not call non forUser Settings.Secure.get* methods

System server should not directly called Settings method such as Settings.Secure.getString()
instead of Settings.Secure.getStringForUser(). The "ForUser" methods allow us to properly
enforce the user's restrictions and it prevents issues like b/163571398.

This CL is part of the effort that changes existing usage of non "ForUser" methods
to using "ForUser" methods. It is supposed to act as a no-op.

BUG: 166312046
Test: builds
Change-Id: I632a243ec916a95437bcd32e5c480ab947eebb4e