Fix leak of RemoteAccessibilityController through SurfacePackage.

When a child SurfacePackage is set on a SurfaceView, the view's
RemoteAccessibilityController calls linkToDeath on the SurfacePackage's
IAccessibilityEmbeddedConnection. The death recipient -- the controller
itself -- is unlinked when the Surface is destroyed -- being notified
through a SurfaceChangedCallback registered on the ViewRootImpl.

There are 2 issues there:
* When the SurfaceView is detached, the SurfaceChangedCallback is
  unregistered, so the unlinkToDeath never happens.
* The Surface holding the SurfaceView may never be destroyed, for
  example in the case of the IME.

SurfacePackages sent over ipc seem to be used in only 2 places
currently:
* For inlined autofill suggestions in the input method. This bug causes
  a serious leak here, since the memory is held in 2 persistent
  processes -- autofill Session data in the system server (referenced
  from the SurfaceViews), and SurfaceViews in the IME.
* The Wallpaper app.

For now, this is fixed by using a WeakRef for the
RemoteAccessibilityController. The root cause will be fixed in a
followup change in a later release.

Fix: 183402294
Test: manual:
 * atest CtsAutoFillServiceTestCases:InlineLoginActivityTest\
   --iterations 5
 * Force gc: adb shell kill -10 $(adb shell pgrep ext.services mockime\
   system_server -d '\ ')
 * Rerun that a couple of times so the binder objects get cleared.
 * Check for leaks of autofill.Session in system_server and
   SurfaceViews or RemoteAccessibilityEmbeddedConnection in
   com.android.cts.mockime.
Change-Id: I5447b173313919507969872bdeb7ff3038152c23