Convert file save toast string to plural

Closes #5961
// FREEBIE

res/values/strings.xml

@@ -167,7 +167,10 @@
         <item quantity="one">Error while saving attachment to storage!</item>
         <item quantity="other">Error while saving attachments to storage!</item>
     </plurals>
-    <string name="ConversationFragment_file_saved_successfully">File saved successfully.</string>
+    <plurals name="ConversationFragment_files_saved_successfully">
+        <item quantity="one">File saved successfully.</item>
+        <item quantity="other">Files saved successfully.</item>
+    </plurals>
     <string name="ConversationFragment_unable_to_write_to_sd_card_exclamation">Unable to write to storage!</string>
     <plurals name="ConversationFragment_saving_n_attachments">
         <item quantity="one">Saving attachment</item>

src/org/thoughtcrime/securesms/util/SaveAttachmentTask.java

@@ -112,7 +112,9 @@ public class SaveAttachmentTask extends ProgressDialogAsyncTask<SaveAttachmentTa
                        Toast.LENGTH_LONG).show();
         break;
       case SUCCESS:
-        Toast.makeText(context, R.string.ConversationFragment_file_saved_successfully,
+        Toast.makeText(context,
+                       context.getResources().getQuantityText(R.plurals.ConversationFragment_files_saved_successfully,
+                                                              attachmentCount),
                        Toast.LENGTH_LONG).show();
         break;
       case WRITE_ACCESS_FAILURE: