Added changes for a11y for the left and right shortcut buttons

This CL aims at making changes for making sure we get the right content
description for the left and right shortcut buttons on the wallpaper
picker screen.

Bug: b/280551132
Test: Tested by building and installing picker on local, and making sure
we get the right text announced. Video attached in bug.

Change-Id: Ic2c845fc75ad94ef187dc1ed484f2635ee00b9f4