Add a button for floating tasks to taskbar (behind a flag)

Test: manual - build launcher3, enable the feature flag on tablet with
               a valid app installed, open an app, observe that there
               is NO button in taskbar because launcher3 doesn't have
               populated config
Bug: 237678727
Change-Id: Ic261608898eb9a9220cffacd0c6060a32c10a74f