Add google analytics event for opening the app (if not already exists)

[JElgar]

[YWKlai]

Hello @JElgar, I had a follow-up discussion with Lizzie and proposed we track the "open app" event as below:

• Detailed data tracking points: plz refer to the attached screenshot.
• Display the data on the lookerstudio for other teams to check. For example, we can display the total open app triggers by iOS and Android and allow people to filter date by date.