A11y changes for notification controller of switch

This CL aims at making a11y changes for notification controller switch
in the wallpaper picker. We added a content description so we announce
the right description when the switch is focussed.

Bug: b/280748132
Test: Tested by building and installing picker on local, and checking
the talkback service. Video attached in bug.

Change-Id: Ifd48ee406a4bafbcf3da208d8f5e49b97be55e7a