Make IMS#clearInsetOfPreviousIme() reliable.

This is a follow-up to my previous CL [1] for Bug 15922840 so that we
can clear the following variables in a more reliable way.
 - PhoneWindowManager#mLastInputMethodWindow
 - PhoneWindowManager#mLastInputMethodTargetWindow

The idea behind CL [2] is that when InputMethodManagerService (IMMS) is
switching from an IME to another IME, IMMS can send a signal to
WindowManagerService (WMS) to remember the current IME's inset so that
the system can continue using it to reduce jank until the new inset is
specified by the next IME.  As summarized in Bug 28781358, however, if
the next IME does not show the window after the IME switch, WMS (or
PhoneWindowManager to be precise) keeps using the previous IME's inset
unexpectedly until the new IME shows its window.  All we have seen in
Bug 15922840 and Bug 26663589 fall into this category.

The idea of this CL is just adding a hidden API to InputMethodManager so
that InputMethodService#clearInsetOfPreviousIme() can surely terminate
the IME transition state managed in PhoneWindowManager, rather than
relying on a hack of calling SoftInputWindow#show() and
SoftInputWindow#hide(), which actually does not work for Bug 26663589.

 [1]: Ib04967f39b2529251e4835c42e9f99dba2cf43f2
      2977eb7b
 [2]: I5723f627ce323b0d12bd7b93f5b35fc4d342b50c
      792faa2c

Note that addressing all the corner cases in [2] still requires lots of
non-trivial change.  Hence this CL focuses only on Bug 26663589 (and
the case we handled in Bug 15922840).

Bug: 26663589
Change-Id: Ib567daa009c1139858dccadcfc6a04465ebecf36