Fix multi-user behaviour for system UI settings (1/2)

Change-Id: I0e434b989c16f5871ce56c7e3041ff3c7a436c73
Make navigation bar ready for per-user configuration.

Also clean it up a bit.

Change-Id: Ic10bd161fce53bf30048a0cb927dcf1e2bfa7774
Merge "Fix multi-user behaviour for system UI settings (1/2)" into cm-10.1
Merge "Make navigation bar ready for per-user configuration." into cm-10.1