Move Desktop running apps support into TaskbarRecentAppsController

- Merged DesktopTaskbarRunningAppsController up into
  TaskbarRecentAppsController, which is now initialized directly
- The old TaskbarRecentAppsController was effectively a no-op
  that was always overridden, so merging the one subclass up makes
  things simpler (especially for the follow up CLs which will add
  support for switching between Running and Recent tasks using
  the same underlying data).

Flag: com.android.launcher3.enable_recents_in_taskbar
Test: TaskbarRecentAppsControllerTest
Bug: 315354060
Change-Id: I8411fb832e5dd3d76201d2694dec0b11bd70bbf9