Enable Buttons and Status Bar settings Multi-user enabled

The main issue is SystemUI process is shared across users, so calling getString(...)
will call UserHandle.myUserId() which will return the user id of the process.
To fix this, all relevant calls should be changed to get*ForUser as well as listen
to UserChanged event so that when the user switches, the settings are updated

Change-Id: I9ac3fbfec1646993028c34789a4299d85079f8f1