Fix a NPE where developer options page crash sometimes.

Developer options page crash for restricted users because the page
removes all preferences while DashboardFeatureProvider tries to
manipulate preferences on screen.

Added a null check to skip changin prefs in DashboardFeatureProvider
because the pref is going to be removed either way.

Change-Id: Ic83fd0dfb2a906605fb1d992d7b36c2163630e89
Fixes: 78655710
Test: robotests