Extract subtype.xml operations as a stateless utility class

This is a mechanical refactoring.  There should be no behavior change.

Bug: 120709962
Test: Manually verified as follows
  1. Build and flash aosp_taimen-userdebug into taimen
  2. Wait until the device fully boots up
  3. adb reboot  # to avoid Bug 121259290
  4. adb root
  5. adb shell cat /data/system/inputmethod/subtypes.xml
      -> make sure the content looks as follows:
         <subtypes>
           <imi id="com.android.inputmethod.latin/.LatinIME">
             <subtype ....>
             <subtype ....>
           </imi/>
         </subtypes>
  6. Open AOSP Keyboard settings
  7. Go to "Appearance & Layouts" -> "Custom input styles"
  8. Remove all layouts
  9. adb shell cat /data/system/inputmethod/subtypes.xml
      -> make sure the file no longer exists
 10. adb shell ls /data/system/inputmethod
      -> make sure the directory no longer exists
 11. adb reboot
 12. adb logcat -s InputMethodManagerService:*
      -> no error message about /data/system/inputmethod
 13. Open AOSP Keyboard settings
 14. Go to "Appearance & Layouts" -> "Custom input styles"
 15. Add English (US) - Dvorak
 16. adb shell ls /data/system/inputmethod
      -> make sure the directory exists and only the system user can
         access it
Test: prebuilts/checkstyle/checkstyle.py -f \
      frameworks/base/services/core/java/com/android/server/inputmethod/AdditionalSubtypeUtils.java
Change-Id: I40942f874fe697049b2f87791f581fbe7ce50fb5