2D Recents: handle keyboard config changes.

While attaching or detaching a physical keyboard, we should handle the
configuration change in RecentsActivity, so that the activity isn't
relaunched and causes abnormal behaviors.

Test: Checked that while keyboard is attached or detached, Recents
behaves normally.
Bug: 32101881

Change-Id: Ibd185aeb48b31dbde032e235fdcaa17fbcdd9c8a