Add test for InputMethodSubtypePreference

Check that InputMethodSubtypePrefernce#compareTo(Preference,Collator)
complies with Comparable contracts.

Fixes: 29571417
Test: Pass SettingsLibTest. Install SettingsLibTest.apk and run test.
  $ adb shell am instrument -w \
      -e class com.android.settingslib.inputmethod.InputMethodSubtypePreferenceTest \
      com.android.settingslib/android.support.test.runner.AndroidJUnitRunner
Change-Id: I96038a7b548469ee34a6f78c75c894f40cd4f338