add 'Cancel' button to location steaming dialog (#3206)

that was just missing
(though cancellation was implicitly always possible using 'back'
or by tapping outside)

src/main/java/org/thoughtcrime/securesms/ShareLocationDialog.java

@@ -23,6 +23,7 @@ public class ShareLocationDialog {
 
       listener.onSelected(shareLocationUnit);
     });
+    builder.setNegativeButton(R.string.cancel, null);
 
     builder.show();
   }