Fix a warning from the binder, step 3.

This patch fixes the following warning:
W/Binder: The following Binder class should be static or
  leaks might occur: com.android.server
  .InputMethodManagerService.MethodCallback

...in hope of removing a possible memory leaks.

Bugs: 5481376, 5461066
Change-Id: I1764090a7059d9bf9e5d90683d7ac190c83415de