Shift transient taskbar to ensure 12dp distance with the bubble bar.

Shift the taskbar to fulfill the 12dp distance if the distance between
the taskbar and the bubble bar is below this threshold.

Fixes: 376929094
Test: TaskbarViewControllerTest
Test: Manual. Test that the transient taskbar shifts properly.
Increased the minimum distance to 120 dp and verified that the taskbar
shifts correctly. Moved bubbles side to side, confirming the taskbar
follows. The return-to-home-screen animation to the hotseat plays as
expected from either side.
Video:
http://recall/-/gx8ASgewUeUS3QYohfrd1J/bJZ8PclmltZu7IupvBFrLn
Test: Manual. Test that the transient taskbar stashes properly.
Increased the minimum distance to 120 dp and verified that the taskbar
shifts correctly. Go to any application. Stash and un-stash the taskbar.
Video:
http://recall/-/gx8ASgewUeUS3QYohfrd1J/hCGRhUgIqvcehOrTQ2Ulvj
Test: Manual. Test that the transient taskbar pin/unpin properly.
Increased the minimum distance to 120 dp and verified that the taskbar
shifts correctly. Pin/unpin the taskbar.
Video:
http://recall/-/gx8ASgewUeUS3QYohfrd1J/f3fy5JKVZAk4wLCpVra4DK
Flag: com.android.wm.shell.enable_bubble_bar

Change-Id: Id4bd713da93053f90d9c2be8503dc93531a03b29