[native] Update the QR login screen to mention the primary device

Summary:
The result of a discussion from D13906.

{F3373001}

Depends on D14052

Test Plan: Opened the QR screen and checked if it looks correctly.

Reviewers: kamil, angelika, bartek, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13946

native/account/qr-code-screen.react.js

@@ -41,7 +41,7 @@ function QRCodeScreen(props: QRCodeScreenProps): React.Node {
     <View style={styles.container}>
       <Text style={styles.heading}>Log in to Comm</Text>
       <Text style={styles.headingSubtext}>
-        Open the Comm app on your phone and scan the QR code below
+        Open the Comm app on your logged-in phone and scan the QR code below
       </Text>
       <View style={styles.qrCodeContainer}>
         <QRCode value={qrCodeURL} size={200} />
@@ -81,6 +81,8 @@ const unboundStyles = {
     fontSize: 12,
     color: 'panelForegroundLabel',
     paddingBottom: 30,
+    textAlign: 'center',
+    width: 300,
   },
   instructionsBox: {
     alignItems: 'center',