Make generation task failed if there is duplicated feature flag name

[r-ralph]

If multiple flags are defined, the current behavior is such that a field with the same name is generated, resulting in a compile-time error.
However, it is difficult to tell what the problem is just by looking at the error, and the error does not occur until the compilation task is executed.
To improve clarity and the speed at which errors are generated, check the feature flag generation task to see if there are duplicate names, and if so, make an error there.

1: Task failed with an exception.
-----------
* What went wrong:
Execution failed for task ':app:generateDebugFeatureFlag'.
> Duplicated feature flag definition found: FEATURE_DUPLICATED_TEST

* Try:
> Run with --info or --debug option to get more log output.
> Get more help at https://help.gradle.org.

[ganadist]

LGTM for now.

And we may migrate these to Gradle Problems API in future.
https://docs.gradle.org/current/javadoc/org/gradle/api/problems/Problems.html