don't finish InstantWalletActivity on scan result

Suggestion for a very minor usability change:
Don't finish() the InstantWallerActivity after the user has scanned a cold storage QR code. This way the activity stays on the back stack and the user only needs to press the "back" key and the "QR code" button again (= 2 clicks) for scanning the next QR code.

Useful for scanning/checking the balances of cold storages in bulk.