Generate signed and aligned apk without putting keystore info in build.gradle. via http://stackoverflow.com/a/20573171/1819810

build.gradle

android {

    // omitting...

    signingConfigs {
        release {
            readKeystoreProperties()
        }
    }

    buildTypes {
        release {
            signingConfig signingConfigs.release
        }
    }

    // omitting...

}

def readKeystoreProperties() {
    def propsFile = rootProject.file('keystore.properties')
    def configName = 'release'
    if (propsFile.exists() && android.signingConfigs.hasProperty(configName)) {
        def props = new Properties()
        props.load(new FileInputStream(propsFile))
        // sign.release.* like `sign.release.storeFile` etc. are defined in `keystore.properties` file
        android.signingConfigs[configName].storeFile = file(props['sign.release.storeFile'])
        android.signingConfigs[configName].storePassword = props['sign.release.storePassword']
        android.signingConfigs[configName].keyAlias = props['sign.release.keyAlias']
        android.signingConfigs[configName].keyPassword = props['sign.release.keyPassword']
        println "===== read keystore properties success ====="
    }
}