Create a shortcut to open bubbles

Allow launching bubbles via a shortcut that is pinned to the home
screen.
When there are no bubbles open, open the bubbles overflow view.
When there are bubbles, open and select the first bubble.
Create an activity in wmshell to handle creating the shortcut and
handling clicking on the shortcut.

Flag: com.android.wm.shell.enable_retrievable_bubbles
Bug: 340337839
Test: manual
Change-Id: If5e2941f265945f78c73fa163f306f1d9bc4d274