Make the BubbleBar flag dynamic in wmshell.

Changing the flag currently requires a reboot, because it is read once when the process starts and never changes.
This change triggers a re-read of the flag value from SystemProperties whenever the bubble bar state listener is registered and unregistered.
This allows setting the flag as part of integration tests for bubble bar.

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: I06bdee8ddb59b3b80ea216205ff107cd84f48237