Add content description to screensaver customization button

This will let users know which screensaver they are customizing.

Fixes: 267582102
Test: flashed device and enabled talkback, opened screensaver settings,
and verified talkback correctly says the name of the screensaver

Change-Id: I9d0d9422f06b7a78efca0e6953b73fe612f79e2a