Show drop target for bubble bar when dragging it

Show where the expanded view will land when dragging bubble bar expanded
view to the other side.

Bug: 313661121
Test: manual
  - test that drop target does not appear when drag does not cross over
    half the screen
  - test that drop target appears when dragging over half the screen
  - test that once drop target appears, it is shown when dragging back
    to initial location
  - test that when drop target is visible, and drag to dismiss target,
    drop target is hidden
  - test that drop target is shown again after dragging out of dismiss
    target and it was showing before

Change-Id: I5503cda9a412b19ac73b3a65a20de50fc99b9e7d