[iOS] Fix QR Code being rendered when the "Send" screen is not in focus

[tuliomir]

In some situations the iOS app identifies that the user is trying to scan a QR Code for sending a transaction, when it's actually looking at the Dashboard. This may be related to recent upgrades to the React Navigation ( #688 ) that may try to pre-render the QR Code screen and ends up activating the QR Reader.

There is already a component effect dedicated to identifying if the QR component should be rendered or not, but it seems to be obsolete as of now.

hathor-wallet-mobile/src/components/QRCodeReader.js

Lines 33 to 40 in 3e301c9

	useEffect(() => {
	/*
	* We need to focus/unfocus the QRCode scanner, so that it doesn't freeze
	* - When the navigation focuses this screen or app becomes active, we set it to `true`
	* - When the navigation moves away or app becomes inactive, we set it to `false`
	*/
	let appStateEvent;
	const focusEvent = navigation.addListener('focus', () => {

Probably this just needs to be adapted.

Steps to reproduce the error

• Load a wallet
• Send a transaction of 0.01 HTR using a QR Code
• When the "Success" modal is being shown, point the camera to the QR Code again
• Dismiss the modal

Expected behavior: the dashboard screen should be displayed and no action taken
Current behavior: while the dashboard screen is being displayed, the QR Code is read again, and the app navigates to the "Send Tokens" screen with no user input.