Add content descriptions to the avatar picker.

This adds content descriptions for all the options in the avatar
picker, including take a photo and choose a photo. An array is
added so that content descriptions can be provided for preselected
images.

Bug: 215134398
Test: Manual, use picker with talkback enabled
Change-Id: Ic317d0e8f841c31c2d4a14e1d037163d9a19a0c4