Add content description to handle in bubble

When a bubble is expanded from bubble bar, it will have the app handle.
Add content description to the handle so users using talkback know what
it is.

Update content description of the handle from "handle" to "app handle"
based on UX feedback.

Bug: 344671270
Flag: com.android.wm.shell.enable_bubble_bar
Test: enable talkback and focus on bubble handle
Change-Id: I22d15b4fcdea4549e9a8b1589ad0d584d4e23af1