SystemUI: Fix navigation bar recreation when applying theme

JB window policy forbids attaching more than one view of
type NAVIGATION_BAR without recreating the entire window.

So instead of detach/recreate/attach the view when applying a theme,
simply avoid touching it during the process, and reposition it
once the reinflated bar is back in place.

Change-Id: I79e9f7da6852c1892f859271ad57561884edc7d2