Settings: Don't crash if we pause too quickly

Notifier is unregistered in every case in pause.
However, if you close the Settings app too quick, the notifier
might not be registered at all, causing an IllegalArgumentException
during the unregistering.

Change-Id: I54141cbfc91d08c28ef524f4155b673ae8dee90e
Settings: fixed battery stats app list

http://jira.omnirom.org/browse/OMNI-493

The check of mAppListGroup getPreferenceCount is no longer working
after we added other options on this page. Add a separate counter to get
back the full possible list of 12 apps

PS3:
IMHO the number of apps should be 10 if we have
MAX_ITEMS_TO_LIST = 10

Change-Id: I679b01deec0f52d4fa2012a31b7b305f8fe176b5