enable silent per recipient ringtone preference

Closes #4957
// FREEBIE

res/xml/recipient_preferences.xml

@@ -14,7 +14,7 @@
                         android:key="pref_key_recipient_ringtone"
                         android:title="@string/recipient_preferences__notification_sound"
                         android:ringtoneType="notification"
-                        android:showSilent="false"
+                        android:showSilent="true"
                         android:showDefault="true"
                         android:persistent="false"/>
 

src/org/thoughtcrime/securesms/RecipientPreferenceActivity.java

@@ -21,7 +21,6 @@ import android.support.v4.app.Fragment;
 import android.support.v4.preference.PreferenceFragment;
 import android.support.v7.app.AlertDialog;
 import android.support.v7.widget.Toolbar;
-import android.text.TextUtils;
 import android.view.MenuItem;
 import android.view.View;
 import android.widget.TextView;
@@ -264,15 +263,21 @@ public class RecipientPreferenceActivity extends PassphraseRequiredActionBarActi
 
       mutePreference.setChecked(recipients.isMuted());
 
-      if (recipients.getRingtone() != null) {
-        Ringtone tone = RingtoneManager.getRingtone(getActivity(), recipients.getRingtone());
+      final Uri toneUri = recipients.getRingtone();
+
+      if (toneUri == null) {
+        ringtonePreference.setSummary(R.string.preferences__default);
+        ringtonePreference.setCurrentRingtone(Settings.System.DEFAULT_NOTIFICATION_URI);
+      } else if (toneUri.toString().isEmpty()) {
+        ringtonePreference.setSummary(R.string.preferences__silent);
+        ringtonePreference.setCurrentRingtone(null);
+      } else {
+        Ringtone tone = RingtoneManager.getRingtone(getActivity(), toneUri);
 
         if (tone != null) {
           ringtonePreference.setSummary(tone.getTitle(getActivity()));
-          ringtonePreference.setCurrentRingtone(recipients.getRingtone());
+          ringtonePreference.setCurrentRingtone(toneUri);
         }
-      } else {
-        ringtonePreference.setSummary(R.string.preferences__default);
       }
 
       if (recipients.getVibrate() == VibrateState.DEFAULT) {
@@ -336,7 +341,7 @@ public class RecipientPreferenceActivity extends PassphraseRequiredActionBarActi
 
         final Uri uri;
 
-        if (TextUtils.isEmpty(value) || Settings.System.DEFAULT_NOTIFICATION_URI.toString().equals(value)) {
+        if (Settings.System.DEFAULT_NOTIFICATION_URI.toString().equals(value)) {
           uri = null;
         } else {
           uri = Uri.parse(value);