Billing code clean up

[PEConn]

This bug is just a grab bag place for me to collect small code cleanup opportunities that I come across.

• Split DigitalGoodsTests into multiple tests (one for each Call).
• Make DigitalGoodsTests use assertItemDetails and createSkuDetailsJson from ItemDetailsTest.
  • Same for the similar methods in PurchaseDetailsTest.
• Standardize on some naming convention for the String constants in *Call and *Details classes.
• Add tests for BillingResultMerger (it is tested indirectly in DigitalGoodsTests, but separate tests would be good).
• Have some guide for when to use package vs public visibility.
• Check the required visibility of the String constants in *Call and *Details.
• See if there's some way to make MockBillingWrapper a little less complex/bitty.
• Add DigitalGoodsTests for when Play Billing calls don't return success.
• Adds tests for each of the *Call methods to ensure they convert the response codes correctly.