Signing Android APK without changing the build.gradle (Same key is used for release and debug). The keystore.json is in the root directory of the project.

build.gradle

android {
    signingConfigs {
        key {
            // Load keystore credentials from a config file
            def credsFilePath = file("../keystore.json").toString()
            def credsFile = new File(credsFilePath, "").getText('UTF-8')
            def json = new groovy.json.JsonSlurper().parseText(credsFile)
            // Save the config into the variables
            storeFile file(json.android.storeFile)
            storePassword json.android.storePassword
            keyAlias json.android.keyAlias
            keyPassword json.android.keyPassword
        }
    }
    buildTypes {
        release {
            // ...
            signingConfig signingConfigs.key
        }
        debug {
            // ...
            signingConfig signingConfigs.key
        }
    }
    // ...
}

keystore.json

{
    "android": {
        "storeFile": "paht/to/file/keystore.jks",
        "storePassword": "storePassword",
        "keyAlias": "keyAlias",
        "keyPassword": "keyPassword"
    }
}