If a tx send is in "max" mode, increasing gasPrice on conf screen should deplete from send value

Repro:
1. Click send
2. Click max mode
3. Fill out other fields
4. Click next
5. Increase gas price a lot

Notice "insufficient funds" error.

Ideally, either:
- Gas info is only editable on one of these two screens
- Both screens respect the "max mode", and so max mode does not allow "insufficient funds" unless the gas total would exceed native currency balance.

Reproduce the flow by sending in max mode, completing the other fields, advancing to the confirmation screen, and substantially increasing gas price. Read the send and confirmation-screen logic involved in gas editing and max mode; done means increasing gas price depletes the send value or otherwise avoids an insufficient-funds error unless total gas exceeds the native-currency balance.

Written by the indexing model from the issue text.