Add a comment about ResultReceiver lifetime.

The root cause of memory leak crbug.com/595613 was that Chromium and
WebView have called InputMethodManager#showSoftInput() with
ResultReceiver thta has a strong reference to application logic class
named ContentViewCore.  In this particular case, ResultReceiver in
question will be copied to InputMethodManagerService and the current
InputMethodService, which means the original receiver object will not be
collected until the copied objects in other processes are GCed.

Probably there might be several ways to oprimize the performance in
Framework side because the corresponding ResultReceiver objects will be
always handled within the Android Framework unless IME developers
explicitly override the following method.
  InputMethodService#onCreateInputMethodInterface()

Anyway, it is probably a bit too late to do such an optimization in N,
and application developers need to support existing versions of Android
anyway.  For N, probably making it clear in JavaDoc would be the only
realistic improvement we can do.

Bug: 27658034
Bug: 27727645
Change-Id: I6fc6b88c91a4b1e0a29e94b99a9f0e35605c01b2