[dash] Fix Android release tutorial to not break builds sans android/key.properties

[artob]

The instructions at https://flutter.io/android-release/ currently have a significant deficiency: after an app's android/app/build.gradle has been modified following these instructions, it is no longer possible to build and run the app without having the android/key.properties file present.

On any development team where only one person acts as release manager, this breaks everybody else's checkouts and flutter run workflow. Moreover, while the fix may be relatively obvious (as per my patch), it wasn't obvious to at least one junior developer who got stuck on the problem for more than a day. It would be good to fix the documentation to eliminate this problem.

Many (eventually all?) Flutter users run into this problem. The workaround is always the same: instructing others to edit their local android/app/build.gradle to comment out the keystore and release configuration. For example, from a quick google:

• https://github.com/iampawan/Flutter-UI-Kit/blob/master/README.md
• Unable to build music player app due to missing key.properties file . iampawan/Flutter-Music-Player#8
• Error when flutter run -d  konifar/droidkaigi2018-flutter#64

I reproduce the error from flutter run below for the benefit of users googling how to fix this problem:

$ flutter run
Using hardware rendering with device Android SDK built for x86. If you get graphics artifacts, consider enabling software rendering with "--enable-software-rendering".
Launching lib/main.dart on Android SDK built for x86 in debug mode...
Initializing gradle...                                       2.6s
Resolving dependencies...                                        
* Error running Gradle:
Exit code 1 from: myapp/android/gradlew app:properties:
Project evaluation failed including an error in afterEvaluate {}. Run with --stacktrace for details of the afterEvaluate {} error.

FAILURE: Build failed with an exception.

* Where:
Build file 'myapp/android/app/build.gradle' line: 29

* What went wrong:
A problem occurred evaluating project ':app'.
> myapp/android/key.properties (No such file or directory)

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 1s


Please review your Gradle project setup in the android/ folder.

[sfshaza2]

Thanks for this PR, @bendiken!

@chinmaygarde, can you review? It's a minor change with a big benefit.

@bendiken, you obviously didn't know we were about to re-structure the dash branch. So, we have two options, assuming Chinmay approves. Either you can resolve the conflicts, or I can create a new branch and copy your edits. Which do you prefer?

[sfshaza2]

Thanks, @jason-simmons! @bendiken, how would you like to proceed regarding the conflicts?

[artob]

Thanks, @jason-simmons! @bendiken, how would you like to proceed regarding the conflicts?

I'll go ahead and resolve and re-submit momentarily 👍

[sfshaza2]

Thanks, @bendiken!