Change the bubble bar flag in wms to a getter.

I accidentally removed it from cl/24904944 before I uploaded the CL.

Bug: 273994695
Test: Manual:
       - turn flag off
       - create bubbles
       - observe floating bubbles are added
       - turn flag on
       - change navigation mode to gesture navigation
       - observe bubble bar becomes visible
Change-Id: If7fee02e026eece1bc9378248a622d601e4f764f