Separate UserSettingsProxy from SettingsProxy

SettingsProxy now contains only settings-related methods that are NOT
'...ForUser()'. This is mainly to remove the dependency on UserTracker
from GlobalSettings, since global setting affect all users, and there is
no need to use the '...ForUser()' methods on it. UserSettingsProxy can
be used for secure and global settings, where per-user settings are
required.

Also:

 - Switch deprecated Settings.Secure.SECURE_FRP_MODE for
   Settings.Global.SECURE_FRP_MODE

 - Fix incorrect access of Settings.Secure.BUGREPORT_IN_POWER_MENU

Test: SystemUITests
Bug: 289353932
Change-Id: I114b737a0c2a2410fa378909fd073930d2266769