Announce when bubble bar expands or collapses

Add a talkback announcement that tells the user when bubble bar expands
or collapses. The announcement includes the bubble name.

Bug: 344670486
Flag: com.android.wm.shell.enable_bubble_bar
Test: expand bubble bar by tapping on it, hear the announcement
Test: collapse bubble bar by tapping on it, hear the announcement
Test: expand/collapse bubble bar from action menu, hear the announcement
Test: use action menu to move expanded bubble bar from one side to the
    other, make sure there is no announcement

Change-Id: I3908bda3eb9e3eb201067fb2c465c3a883ce73d4