settings: prevent NPE if device doesn't support ActionBar

If the device FEATURE_ACTION_BAR (the device hasn't it enabled or a theme has disabled it), getActionBar()
will return a null reference. Just prevent to use that invalid ref.

JIRA: BUGDUMP-23210
https://jira.cyanogenmod.org/browse/BUGDUMP-23210
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>

Change-Id: I210867b10193471c49e444c27c47e9de6baee822