Warn if IMM#showSoftInputUnchecked() is called

With this CL, an @hide method InputMethodManager#showSoftInputUnchecked
is marked to be deprecated and starts showing a warning message in logcat
when it gets called to tell application developers who are still using
old implementation fo android.support.v7.widget.SearchView that they
need to switch to support library ver. 26.0 or later version.

Other than that there is no behavior change in this CL.

Test: Manually verified as follows:
       1. Flash an OS image
       2. Complete the setup wizard (if any).
       3. In N MR-1 AOSP repository
          frameworks/support/gradlew -p frameworks/support support-v7-demos:assemble
       4. adb install out/host/gradle/frameworks/support/support-v7-demos/build/outputs/apk/support-v7-demos-debug.apk
       5. adb logcat -s InputMethodManager
       6. adb shell am start -n com.example.android.supportv7/.Support7Demos
       7. Tap AppCompat -> Action Bar -> Action Bar Usage
       8. Tap the magnifier icon.
       9. Make sure that the software keyboard shows up.
      10. Make sure that in logcat a warning that showSoftInputUnchecked
          is going to be removed is shown.
Fixes: 36015425
Change-Id: If01316a0c2a210f9ea03b53700d0ef651955ba9c