secure-properties-provider.gradle

def getSecureProperty(String propertyName) throws MissingSecurePropertyException {
    def property = project.findProperty(propertyName) ?: System.getenv(propertyName)
    if (property != null) {
        return property
    } else {
        throw new MissingSecurePropertyException(propertyName)
    }
}

class MissingSecurePropertyException extends RuntimeException {

    private final String property

    MissingSecurePropertyException(String property) {
        super()
        this.property = property
    }

    @Override
    String getMessage() {
        return "The property \"${property}\" is not found. " +
                "Please add this property to ~/.gradle/gradle.properties."
    }

}

ext {
    getSecureProperty = this.&getSecureProperty
}

Также нужно включить фичу buildConfig

android {
    buildFeatures {
        buildConfig = true
    }
}

Но можно не создавать отдельный файл

Пример для gradle.kts

android {
    defaultConfig {
        buildConfigField(
            type = "String",
            name = "MAPKIT_API_KEY",
            value = "\"${project.findProperty("yandexMapKitApiKey")}\"",
        )
    }
}