Fix zoom gesture crash; request camera permission upon tap if not granted.

Based on patch in googlesamples#145.

Author: pchx

visionSamples/barcode-reader/app/src/main/java/com/google/android/gms/samples/vision/barcodereader/BarcodeCaptureActivity.java

@@ -137,6 +137,7 @@ public void onClick(View view) {
             }
         };
 
+        findViewById(R.id.topLayout).setOnClickListener(listener);
         Snackbar.make(mGraphicOverlay, R.string.permission_camera_rationale,
                 Snackbar.LENGTH_INDEFINITE)
                 .setAction(R.string.ok, listener)