Make the bubble bar flag dynamic in Launcher.

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 Taskbar is recreated.
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: I277ea6aa2c8d3ab04094537bf4cd31630310dfec