Eliminate out-of-sync IMM#mFullscreenMode error

As explained in the commit message of my previous CL [1], we have
had a design issue in how to notify the full-screen mode change
from the IME to InputMethodManager running in the target application.

Histrically we have done this by using hooking the following IPC
from the IME to the target application.

  InputConnection#reportFullscreenMode()

However, since we also want InputConnection to be deactivated in some
situations such as the when the target application is no longer
focused. In other words, InputConnection is not a reliable way to
notify something.

As a result, we have suffered from many stale state issues.
Bug 21455064 and Bug 28157836 are such examples.  In Android N, we
introduced yet another hack to work around those issues, but it is
really time to fix the protocol design instead.

The new strategy is to rely on internal IPCs provided by
InputMethodManager to deliver such critical notifications from one
process to the other. This is actually more natural because our goal
is to make sure that InputMethodManager#isFullscreenMode() always
returns the latest value as long as the caller is the focused
application.

For backword compatibility, applications that are monitoring
this callback should continue working, as InputMethodManager emulates
the previous behavior.  However, as updated in JavaDoc, IMEs are no
longer allowed to invoke InputConnection#reportFullscreenMode(),
which should be OK because even on previous releases IMEs should rely on
InputMethodService#updateFullscreenMode() instead.

 [1]: Iba184245a01a3b340f006bc4e415d304de3c2696
      1544def0

Fixes: 28406127
Test: Make sure Bug 21455064 is still fixed.
       1. Input some words in extract mode.
       2. Select a word.
       3. Perform copy.
       4. Select a word.
       5. Rotate the device.
       6. Try to select a word.
       7. Make sure he word is selected and action mode starts.
Test: Make sure Bug 28157836 is still fixed.
       1. Rotate device to landscape mode.
       2. Tap on EditText and start full screen extracted mode.
       3. Rotate device to portrait mode.
       4. Long press to start action mode.
       5. Make sure Action mode gets started.
Test: `adb shell dumpsys input_method` to make sure that fullscreen
      state is synchronized across the app, IMMS, and the IME.
Change-Id: If23e7c7c265ab3dfb48c2fb6fdb361b17d22c594