Short example of storing API Keys for an Android repo.

.gitignore

local.properties
# Don't forget to ignore this from your source control

build.gradle

android {
    Properties properties = new Properties()
    properties.load(project.rootProject.file("local.properties").newDataInputStream())

    defaultConfig {
        buildConfigField("String", "MY_API_KEY", properties.getProperty("APIKey"))
    }
}

local.properties

APIKey="MyAPIKeyHere"

MainActivity.kt

class MainActivity {
    val apiKey = BuildConfig.MY_API_KEY
}