Back button behavior doesn't make sense in some situations

[SyntaxPolice]

Basically, there's a stack of activities where it does not seem to make sense to ever go back to the previous one.

• Main activity
• Scanning activity (back behavior makes sense)
• Confirmation activity: "Are you sure you want to log in". There's no "no" option, so if not, the user will need to go back. Heading back to the scanner at this point is probably not right since they didn't want to log in. Go to the main activity.
• Pincode activity: If the user hits "back", it's only because they don't want to log in. Go to the main activity.
• Summary activity: you have successfully logged in". Using the back button here never really makes sense. Re-entering the pincode will result in an expired challenge error. It sends you to the pincode activity with no way to get back to some place meaningful except by going back until you reach the home screen. Go to the main activity.

I might suggest that most of these activities can finish() when they move onto the next one in the stack since it's not meaningful to revisit them.