Exclude empty and custom screens when centering wallpaper

If the user has CM Home or Google Now enabled then getChildCount()
will include those as well.  We need to exclude those screens when
determining if the wallpaper should be centered for one screen.

Change-Id: I702feb752b37d8c714352a2e18f0c61e6f640ba9
Merge remote-tracking branch 'cm/cm-11.0' into cm-11.0