Woo/grouped products small fixes

[anitaa1990]

While working on woocommerce/woocommerce-android#2630, I noticed a few issues when displaying a group of products. This PR attempts to fix those issues.

• The list of groupedProductIds should be Long since the remoteProductId is Long everywhere in the app. So modified WCProductModel to reflect this change in e2b7a4f.
• When fetching the product details for a list of groupedProductIds, I noticed that we delete the previously stored products from the local db before adding these products. This means in the woo app, the Products tab only displays the groupedProducts and we need to manually refresh the app to fetch the list of products again. So I have added logic to delete the locally stored products only if the remoteProductIds are null.

To test

• Verify that Fetching products, fetching a single product and updating grouped products works as expected.

[0nko]