moved out init functions to App and removed initial loading of multiple WalletScreens

[jinchung]

• WalletScreen being loaded multiple times because of "initial screens" were all pointing to WalletScreen
• We can just load the screens but the initial screen will be resolved without having to set all the screens to WalletScreen initially
• Also moved out the 'wallet initialization' logic that should only happen once to the App level instead of WalletScreen level