Generalize ActivityTracker to support RecentsWindowManager

Continued quick switch couldn't work using existing infrastructure until RecentsWindowManager could be properly tracked by ActivityTracker. Refactored ActivityTracker into ContextTracker to support this.

Also refactored ActivityInitListener into ContextInitListener to fit the updated pattern.

Flag: com.android.launcher3.enable_fallback_overview_in_window
Fixes: 366023051
Test: RecentsWindowSwipeHandlerTestCase; Attempted continued quick switch from home and launched app
Change-Id: Ic38ebf3611ef22fbfd1ddeb79d72d8a3621940a0