qml/android: set max screen brightness when displaying QR codes

In some cases this makes it much easier to successfully scan a QR code.
I was trying to scan a PSBT using a laptop camera from my phone screen for 2 minutes, until I realised the screen brightness was the issue.  o.O

electrum/gui/qml/components/controls/QRImage.qml

@@ -24,7 +24,7 @@ Item {
         source: qrdata && render ? 'image://qrgen/' + qrdata : ''
         visible: !isTextState
 
-        Rectangle {
+        Rectangle {  // container for logo inside qr code
             visible: root.render && _qrprops.valid
             color: 'white'
             x: (parent.width - width) / 2
@@ -75,4 +75,13 @@ Item {
         }
     }
 
+    onVisibleChanged: {
+        if (root.visible) {
+            // set max brightness to make qr code easier to scan
+            AppController.setMaxScreenBrightness()
+        } else {
+            AppController.resetScreenBrightness()
+        }
+    }
+
 }