Fix a warning from the binder.

This patch fixes the following warning:
W/Binder: The following Binder class should be static or
  leaks might occur: android.view.textservice
  .SpellCheckerSession.InternalListener

...in hope of removing a possible memory leaks.
However in my tests it does nothing at all for the memory leak
in bug 5461066, so it was probably unrelated.

Change-Id: Id776665e9483121d22cc91e8d2d9f32e3413ec96
Bugs: 5481376,5461066