Add a flag for the bubble bar

This is an UnreleasedFlag which is only available in SysUI, so we
set it on Bubbles via BubblesManager which is part of SysUI.

Changes to support bubble bar will happen behind this flag.

Bug: 253318833
Bug: 256873975
Test: manual - enable flag and add a bubble on a large screen device
             - observe that the bubbles are pinned to the bottom
Change-Id: I0d86a71b66db683837bf639d9067d92703b4364e