Address memory leak in EdgeBackGestureHandler.

EdgeBackGestureHandler listens to changes in GestureInteractor by
collecting the blocked activity flow. Previously, this flow was
untracked and therefore not cleaned up when the
EdgeBackGestureHandler was discarded. This changelist addresse
this by retaining the returned Job and canceling it when the
EdgeBackGestureHandler is not enabled.

Test: traced memory usage in hprof to ensure only a single
      instance is retained.
Fixes: 355835858
Flag: EXEMPT bugfix.
Change-Id: I0c1519326cadc4d048e98a7960e3f8bcc2b0ced4