Use the type not the null check to identify a shortcut bubble

The check here was looking for a shortcut bubble from bubble
anything. It did this by checking the flag + the presence of
a shortcut on the bubble. However, this would be true for
chat bubbles as well -- even if the chat isn't based on a
shortcut (apps could be using the pending intent API).

This led to a bug where we'd use the wrong content to
populate the bubble & the app would misbehave. This would
only occur when bubble anything is enabled.

Flag: com.android.wm.shell.enable_bubble_anything
Bug: 385361517
Test: manual - enable bubble anything and expand a
               signal bubble
             => observe that it expands correctly
Test: WIP -- extracting TaskView.Listener to make make it
      more testable
Change-Id: Id3e95dd9e12ad4a19aacf5e6691b145ac0046680