Reading the value of keystore from file.

readKeystore.gradle

allprojects {
    afterEvaluate { project ->
        // you should define property in keystore.properties like this :
        // storePassword="..."
        // keyPassword="..."
        // keyAlias="..."
        // storeFile="..."
        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))
            android.signingConfigs[configName].storeFile = file(props['storeFile'])
            android.signingConfigs[configName].storePassword = props['storePassword']
            android.signingConfigs[configName].keyAlias = props['keyAlias']
            android.signingConfigs[configName].keyPassword = props['keyPassword']
        }
    }
}