feat: add unitTesting gradle variable

[edusperoni]

this adds the unitTesting gradle variable at build time.

Users can then do:

// app.gradle
android {
  defaultConfig {
    manifestPlaceholders = [isUnitTesting:"${unitTesting}"]
  }
}

// AndroidManifest.xml
	<application
                ....
		android:usesCleartextTraffic="${isUnitTesting}"
        >

More advanced users that are using things like android:networkSecurityConfig="@xml/network_security_config" can do:

def securityConfig = "@xml/network_security_config";
if(unitTesting.toBoolean()) { // toBoolean() is important!
  securityConfig = "@xml/my_insecure_security_config";
}
// app.gradle
android {
  defaultConfig {
    manifestPlaceholders = [securityConfig :"${securityConfig}"]
  }
}

// AndroidManifest.xml
	<application
                ....
		android:networkSecurityConfig="${securityConfig}"
        >