Call InputEventReceiver.dispose from the right thread

TaskPositioner attaches the InputEventReceiver to the animation thread.
This CL makes it to dispose the receiver in the same thread to avoid
race conditions.

This CL also makes InputEventReceiver.finalize call dispose from the
right thread.

Bug: 122054478
Fix: 122096091
Test: 1. Make a task enter free-form mode.
      2. Drag and drop the task.
      3. Check if InputEventReceiver.dispose and finishInputEvent are
         executed in the same thread.

Change-Id: I2f8831e7fccca4f96562f2abe4962811339d02e9