Espresso Read String File Value

You should use InstrumentationRegistry.getContext() instead of InstrumentationRegistry.getTargetContext() and then use appropriate package for generated R file (by default it will append .test to your package):

Resources resources = InstrumentationRegistry.getContext().getResources();
String name = resources.getString(com.your_package.test.R.string.product_name);

Pay attention to test part in resource name.