Improve wording of joined contact notification

Fixes #6757
Closes #6764
// FREEBIE

res/values/strings.xml

@@ -417,7 +417,7 @@
     <string name="MessageRecord_s_called_you">%s called you</string>
     <string name="MessageRecord_called_s">Called %s</string>
     <string name="MessageRecord_missed_call_from">Missed call from %s</string>
-    <string name="MessageRecord_s_joined_signal">%s joined Signal!</string>
+    <string name="MessageRecord_s_joined_signal">%s is on Signal!</string>
     <string name="MessageRecord_you_set_disappearing_message_time_to_s">You set disappearing message time to %1$s.</string>
     <string name="MessageRecord_s_set_disappearing_message_time_to_s">%1$s set disappearing message time to %2$s.</string>
     <string name="MessageRecord_your_safety_number_with_s_has_changed">Your safety number with %s has changed.</string>
@@ -594,7 +594,7 @@
     <string name="ThreadRecord_called_you">Called you</string>
     <string name="ThreadRecord_missed_call">Missed call</string>
     <string name="ThreadRecord_media_message">Media message</string>
-    <string name="ThreadRecord_s_joined_signal">%s joined Signal!</string>
+    <string name="ThreadRecord_s_is_on_signal">%s is on Signal!</string>
     <string name="ThreadRecord_disappearing_message_time_updated_to_s">Disappearing message time set to %s</string>
     <string name="ThreadRecord_safety_number_changed">Safety number changed</string>
     <string name="ThreadRecord_your_safety_number_with_s_has_changed">Your safety number with %s has changed.</string>

src/org/thoughtcrime/securesms/database/model/ThreadRecord.java

@@ -98,7 +98,7 @@ public class ThreadRecord extends DisplayRecord {
     } else if (SmsDatabase.Types.isMissedCall(type)) {
       return emphasisAdded(context.getString(org.thoughtcrime.securesms.R.string.ThreadRecord_missed_call));
     } else if (SmsDatabase.Types.isJoinedType(type)) {
-      return emphasisAdded(context.getString(R.string.ThreadRecord_s_joined_signal, getRecipients().getPrimaryRecipient().toShortString()));
+      return emphasisAdded(context.getString(R.string.ThreadRecord_s_is_on_signal, getRecipients().getPrimaryRecipient().toShortString()));
     } else if (SmsDatabase.Types.isExpirationTimerUpdate(type)) {
       String time = ExpirationUtil.getExpirationDisplayValue(context, (int) (getExpiresIn() / 1000));
       return emphasisAdded(context.getString(R.string.ThreadRecord_disappearing_message_time_updated_to_s, time));