build.gradle

def getVersionCode = { ->
    def code = project.hasProperty('versionCode') ? versionCode.toInteger() : 9999999
    println "versionCode is $code"
    return code
}

def getCodePushKey = { ->
    def key = project.hasProperty('codePushKey') ? codePushKey : ""
    println "codePushKey is $key"
    return "\"${key}\""
}

android {
...
    defaultConfig {
        ...
        versionCode getVersionCode()
        ...
    }

    buildTypes {
        debug {
            buildConfigField "String", "CODEPUSH_KEY", '""'
        }

        release {
            buildConfigField "String", "CODEPUSH_KEY", getCodePushKey()
            ...
        }
    }
...

Fastfile.rb

def normalize_path(path)
  File.expand_path(path, '..')
end

def read_file(path)
  unless File.file?(path)
    puts "You must provide '#{path}' file"
    exit 1
  end

  File.read(path)
end

def write_file(path, data)
  File.write(path, data)
end

def get_version(path)
  path = normalize_path(path)

  version = read_file(path).to_i
  version = version + 1
  write_file(path, version)

  version
end

def check_keystore(path)
  path = normalize_path(path)

  unless File.file?(path)
    puts "You must provide '#{path}' keystore file for release or next version"
    exit 1
  end
end

version_file = 'fastlane/version'
keystore_file = 'fastlane/release.jks'

platform :android do
  private_lane :build do |options|
    version_code = get_version(version_file)
    check_keystore(keystore_file)

    gradle(
      task: 'clean',
      project_dir: 'android'
    )
    gradle(
      project_dir: 'android',
      properties: {
        versionCode: version_code,
        codePushKey: options[:codePushKey]
      },
      **options[:gradle]
    )
  end
  
  desc 'Build release version'
  lane :build_release do |options|
    build(
      gradle: {task: 'assemble', build_type: 'Release'},
      **options
    )
    sign_apk(
      keystore_path: keystore_file,
      alias: 'release',
      tsa: 'http://timestamp.digicert.com',
      **keystore_keys
    )
    zipalign(apk_path: lane_context[SharedValues::SIGNED_APK_PATH])
  end
  
  desc 'Upload Google Play release'
  lane :release do
    build_release(codePushKey: push_code_keys['android']['release'])
    supply(
      apk: lane_context[SharedValues::SIGNED_APK_PATH],
    )
  end
  # ...
end

version

1