Let the absence of subtype.xml mean there is no additional subtype

Historically, we have always created a stub subtype.xml for each user
even when there is no additional subtype.  Instead of having such a
stub XML files, no subtype will be represented by the absence of
subtype.xml with this CL.

If there has already been such a stub XML, it will be deleted
automatically when
InputMethodFileManager#writeAdditionalInputMethodSubtypes() gets
called next time.  Note that this method is called from
InputMethodManagerService#onFinishPackageChangesInternal(), which is
called when some IME package is updated.

This is completely about how internal data is persisted.  There should
be no developer-visible behavior change.

Fix: 121223050
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
Change-Id: If06e1b5967443157d1a7606190d721ae8983be08