Hide InputMethodManger.showSoftInputUnchecked() from Q+ apps

This is a follow up CL to my previous CL [1], which declared that
InputMethodManger.showSoftInputUnchecked() was deprecated and should
have never be used any more.

At that time, however, we could not simply remove that hidden API
because android.support.v7.widget.SearchView had been used it via
reflection and the reflection code was already statically linked into
many apps in the ecosystem.  What we could do at that time was getting
rid of this hidden API dependency from support lib v26 [2] and making
sure that a clear warning was shown in logcat if apps were still using
older support lib [3].

Almost two years later now, it's probably time to step forward towards
true deprecation of this hidden API.  With this CL, apps that target
Q+ can no longer call this method.

 [1]: I20983e4ce1d625e098a8c2335ce75994cfa43235
      5cfc1b4c
 [2]: Iaac4673a85c141c787b96fb39806346ccfb85fb3
      fd44fffced7cce36c1ce0d62dfe2542aa6a7c6e9
 [3]: If01316a0c2a210f9ea03b53700d0ef651955ba9c
      101ae464

Bug: 36015425
Fix: 123768499
Test: make -j checkbuild
Change-Id: Ie7362f62f1e947ee5a21b90f5150c69f2970d73d