Allow set custom build id when variantMapsEnabled is false

[SilentCatD]

Hi there,

Recently, we encountered a problem with integration with Shorebird CodePush, the reason is the BUILD_ID keep changing when we perform code patching.

This entails cleaning the Flutter project, causing the generate NewRelicConfig gradle task to be run and generate a new config with new BUILD_ID.

This is a problem for codepush, because changing in this generated file is considered native change.

Please refer to these issue for more information:

https://github.com/newrelic/newrelic-flutter-agent/issues/123 https://github.com/shorebirdtech/shorebird/issues/755

I have also filed a feature request ticket in newrelic support: NR-334435

Nevertheless, this PR will allow for specify custom build id inside app/build.gradle file, along with original newrelic config.

Currently, in our Flutter app, I use it like this:

// app/build.gradle file

newrelic {
    variantMapsEnabled = false
    buildId = "${flutterVersionName}.${flutterVersionCode}"
}

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