Move PZP ConfigActivity off launcher menu.

Jira issue: WP-831

The PZP ConfigActivity is now accessed from a 'Settings ...' menu option
on the WidgetList activity, and the launcher entry is removed.

Also some cosmetic improvement to the WidgetList activity to show an
indication when the list is empty.
Merge pull request #502 from paddybyers/android-pzp-config

Move PZP ConfigActivity off launcher menu.