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.