WMShell: use the aconfig flag for bubble bar

Test: make & enable / disable the flag to see if it works
      adb shell device_config put multitasking
          com.android.wm.shell.enable_bubble_bar true
Test: PlatformScenarioTests:
      android.platform.test.scenario.sysui.bubble.BubbleBarTest
Bug: 286246694
Change-Id: I832b452164f565758827e8ef29b5ccffdbb4f41d