Fix stale InputMethodManager#mFullscreenMode.

The current mechanism to sync InputMethodService#mIsFullscreen to
InputMethodManager#mFullscreenMode is really fragile because
  1. Currently the state change is notified via
     InputConnection#reportFullscreenMode(), where InputConnection is
     designed to be valid only while the IME has input focus to the
     target widget.
  2. In favor of performance InputMethodService (IMS) calls
     InputConnection#reportFullscreenMode() only when #mIsFullscreen
     changed.  If InputConnection#reportFullscreenMode() failed, there
     is no recovery mechanism.
  3. Screen oriantation change is likely to cause Window/View focus
     state change in the target application, which is likely to
     invalidate the current InputConnection.

What our previous workaround [1] did for Bug 21455064 was actually
relaxing the rule 1 only for InputConnection#reportFullscreenMode().
However, my another CL [2] made the lifetime check of InputConnection a
bit more strict again, which revived the issue as Bug 28157836.

Probably a long-term fix would be to stop using InputConnection to sync
that boolean state between IMS and the application.  However, it's too
late to do such a refactoring in N, hence this CL relaxes the rule 1
again keeping it as secure as possible.

The idea is that we allow InputConnection#reportFullscreenMode() to
update InputMethodManager#mFullscreenMode regardless of whether
InputConnection is active or not, as long as the InputConnection is
bound to the curent IME.  Doing this as a short-term solution is
supporsed to not introduce any new risk because the active IME is
already able to mess up the InputMethodManager#mFullscreenMode by
calling InputConnection#reportFullscreenMode() on any other active
InputConnection.  Bug 28406127 will track the long-term solution.

 [1]: Id10315efc41d86407ccfb0a2d3956bcd7c0909b8
      da589dff
 [2]: If2a03bc84d318775fd4a197fa43acde086eda442
      aaa38c9f

Bug: 28157836
Change-Id: Iba184245a01a3b340f006bc4e415d304de3c2696