Explain why SMS fallback is disabled when it is

res/values/strings.xml

@@ -690,7 +690,8 @@
     <string name="preferences__use_the_data_channel_for_communication_with_other_textsecure_users">
         Increase privacy and avoid SMS fees by using the data channel for communication with other TextSecure users
     </string>
-    <string name="preferences__allow_sms_fallback">Allow SMS Fallback</string>
+    <string name="preferences__allow_sms_fallback">SMS Fallback</string>
+    <string name="preferences__allow_sms_fallback_disabled_reason">TextSecure is currently your default SMS app. Please set another default SMS app first to change this preference.</string>
     <string name="preferences__send_and_receive_sms_messages_when_push_is_not_available">Send and receive SMS messages when push is not available</string>
     <string name="preferences__refresh_push_directory">Refresh Push Directory</string>
 

src/org/thoughtcrime/securesms/ApplicationPreferencesActivity.java

@@ -210,8 +210,10 @@ public class ApplicationPreferencesActivity extends PassphraseRequiredSherlockPr
       if (Util.isDefaultSmsProvider(this) || !TextSecurePreferences.isPushRegistered(this)) {
         allowSmsPreference.setEnabled(false);
         allowSmsPreference.setChecked(true);
+        allowSmsPreference.setSummary(R.string.preferences__allow_sms_fallback_disabled_reason);
       } else {
         allowSmsPreference.setEnabled(true);
+        allowSmsPreference.setSummary(R.string.preferences__send_and_receive_sms_messages_when_push_is_not_available);
       }
     } else {
       if (TextSecurePreferences.isInterceptAllMmsEnabled(this) ||
@@ -220,8 +222,10 @@ public class ApplicationPreferencesActivity extends PassphraseRequiredSherlockPr
       {
         allowSmsPreference.setEnabled(false);
         allowSmsPreference.setChecked(true);
+        allowSmsPreference.setSummary(R.string.preferences__allow_sms_fallback_disabled_reason);
       } else {
         allowSmsPreference.setEnabled(true);
+        allowSmsPreference.setSummary(R.string.preferences__send_and_receive_sms_messages_when_push_is_not_available);
       }
     }
   }