Dispose mPointerEventDispatcher on UiThread

The function InputEventReceiver::dispose must be called on the looper thread. This is documented in InputEventReceiver::dispose:


   /**
     * Disposes the receiver.
     * Must be called on the same Looper thread to which the receiver is attached.
     */
    public void dispose() {
        dispose(false);
    }

That expectation is not currently met in the DisplayContent code.

Update the code so that it conforms to the expectation.

This will allow us in subsequent CLs to crash whenever dispose is called on the wrong thread.

Bug: 376713684
Bug: 311142655
Test: presubmit
Flag: EXEMPT bugfix
Change-Id: I86ffb14fee7c3e7b31a47205f84572ae640d234b