Sample app checkout colors are not configurable

[kyle-schellen]

The sample app has a configuration bug that prevents custom checkout colors from being applied reliably.

Changing the color values in the sample checkout configuration does not consistently propagate to the native checkout sheet. This affects the ability to test and demonstrate custom colors configuration for iOS and Android.

We should fix the sample app configuration so custom color overrides are respected.

[kyle-schellen]

Moved tracking to Shopify/checkout-sdk-issues#625: https://github.com/Shopify/checkout-sdk-issues/issues/625