Rename SlicePurchaseWebInterface to DataBoostWebServiceFlow

Also remove duration parameter from notifyPurchaseSuccessful
Changed to be consistent with the TS.43 specs

Test: manual verify no regression of purchase behavior
Test: atest CarrierDefaultAppTest
Bug: 283210989
Change-Id: I6d93ca848495e1e2657518d12b61f4ab9796a035