Fix ImeSubtypeListItem#compareTo()

It turns out that ImeSubtypeListItem#compareTo() does not satisfy the
contract of Comparable#compareTo(), which can trigger
IllegalArgumentException from Collections.sort() depending on the
runtime condition.

This CL makes it clear that two instances of ImeSubtypeListItem will be
compared with with those fileds in the following order.

  1. ImeSubtypeListItem#mImeName
  2. ImeSubtypeListItem#mSubtypeName
  3. ImeSubtypeListItem#mIsSystemLocale
  4. ImeSubtypeListItem#mIsSystemLanguage

Bug: 34255739
Test: adb shell am instrument -w -e class com.android.internal.inputmethod.InputMethodSubtypeSwitchingControllerTest com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I47f902cc8f5873926d238c30e462d08d7dbebcf7