Replace HashSet with ArraySet in InputMethodSubtypeSwitchingController

There should be no user-visible bahavior change because
InputMethodSubtypeSwitchingController relies only on the following
three operations:
  * Collection#add()
  * Collection#remove()
  * Collection#contains()

There should be no behavior difference between HashSet and ArraySet
for those operations.

Bug: 119839847
Test: Manually verified as follows:
  1. Open Settings.
  2. Go to System -> Languages & input -> Languages
  3. Tap "Add a language"
  4. Select "Afrikaans" -> Suid-Afrika
  5. make -j SoftKeyboard
  6. adb install -r $OUT/system/app/SoftKeyboard/SoftKeyboard.apk
  7. adb shell ime enable com.example.android.softkeyboard/.SoftKeyboard
  8. Tap the keyboard icon on the navigation bar.
  9. Make sure that items are sorted in the following order:
      * English (US) - Android Keyboard (AOSP)
      * Afrikaans - Android Keyboard (AOSP)
      * English (United States) - Sample Soft Keyboard
Change-Id: Ib09826962dacea1f5edfc37012f4794851288f47