Fix Talkback wrong label announced going from App to Overview

To fix the incorrect title information being spoken after a transition state ends, a new function named getTitle is introduced in the LauncherState. This function provides the correct title for the transition state. The default accessibility label is set to R.string.home_screen. All Apps and Overview states override the getTitle function to ensure the correct accessibility label for their respective states.

Fix: 265892510
Flag: EXEMPT bugfix
Test: Manual with Talkback. With Talkback enabled, navigate from App to Overview.
Change-Id: I96b820f310f8b778a149709bca687a5912386246