Fixing regression in Recents preloading.

- Changes in the CommandQueue callbacks resulted in the default
  implementation of preloadRecentApps() to be called instead of the
  actual implementation in the Recents component.
- Removing extraneous methods in the interface called from other parts of
  SystemUI.

Bug: 38390446
Test: Launch Recents, ensure that preloading is hit first on touch down on
      the button

Change-Id: I28f7ac47eafa76a53343505f3352760e6510c213