Improve Flutter device integration test coverage

[asimihsan]

Introduction

A Flutter device integration test uses either a simulator or a real device to execute an end-to-end integration test. The full Flutter app is compiled and then a "driver" on the device is able to receive commands from the integration test, e.g. "get the text of this TextField" and "tap this Button".

Theoretically you should be able to run Flutter device integration tests on AWS Device Farm devices but that is out of scope of this issue.

Problem statement

Currently there is a single Flutter device integration test that checks when you generate a password it's not the same as the previous password. We should at least test that the settings switches work and generate different kinds of passwords.

Acceptance criteria

• Integration test for settings affecting the password.
• Integration test confirming you can use the help screen.
• Other quick integration tests you can think of.