Only register RecentTasksChangedListener if flag is enabled

Also unregister it if the feature becomes disabled (currently
this is only done in tests but in the future there will be a
user setting as well).

Added tests to verify getTasks() is not called while both
canShowRunningApps and canShowRecentApps are false.

Bug: 354087794
Test: TaskbarRecentAppsControllerTest
Flag: com.android.launcher3.enable_recents_in_taskbar
Flag: com.android.window.flags.enable_desktop_windowing_taskbar_running_apps
Change-Id: I3123501e7584ad7f877e13294454556501d2dbc8