Fix Settings crash when setting a null ringtone

Ringtone picker may callback a null ringtone Uri
if users select None.

This change pass null ringtone Uri to RingtoneManager
and return.

Bug: 232502532
Bug: 221041256
Test: maunal
      Settings - Sound & Vibration -> Phone ringtone
      -> My Sounds -> None
Change-Id: I044b680871472a3c272f6264c4ef272df542112e

src/com/android/settings/DefaultRingtonePreference.java

@@ -46,6 +46,11 @@ public class DefaultRingtonePreference extends RingtonePreference {
 
     @Override
     protected void onSaveRingtone(Uri ringtoneUri) {
+        if (ringtoneUri == null) {
+            setActualDefaultRingtoneUri(ringtoneUri);
+            return;
+        }
+
         String mimeType = getContext().getContentResolver().getType(ringtoneUri);
         if (mimeType == null) {
             Log.e(TAG, "onSaveRingtone for URI:" + ringtoneUri