Fix crash on Storage and Privacy page

The implementation in PreferenceGroup is changed,
contentEquals doesn't accept null value.

So, we won't call findPreference when key of controller
is null.

Test: robotest, maunal test
Fixes: 122805831

Change-Id: Ia6df5c536ca5a32448f66b5308e7ec2334f1fe7b
(cherry picked from commit 3314cd05d5b97368e7c8f7ea7470f7321fbcdf95)