Bubble handle is not focusable with talkback in overflow

Bubble handle is not shown in the overflow view. But it was still
focusable with talkback turned on.
Updating the visibility of the handle to ensure that talkback does not
try to focus on it.

Bug: 344674602
Flag: com.android.wm.shell.enable_bubble_bar
Test: enable talkback and open overflow in bubble bar, ensure that
  handle can't be navigated to with swipe to next gesture
Change-Id: If9f9ea3bb583358b6128ac9cb1b024ea33da871a