Make partially visible widget pages not important for accessibility.

1. If the lockscreen has more than one widet, the left and right widget
   pages are partially on the screen and visible. Therefore they may
   take accessibility focus so the user will hear a prompt that he is
   on the partially visible widget page but will not be able to interact
   with it - confusing. This change makes partially shown windget frames
   not important for accessibility.

bug:7454355

Change-Id: I475b21e215474a4421acaec485cde64dc4e2aee5