Rename entitlement callback to adhere to API guidelines.

The method for OnTetheringEntitlementResultListener needs to be
onTetheringEntitlementResult in order to meet our API guidelines:

"Use Listener when there is a single callback method and there will
never be any others. It should be an interface whose name is the same
as the callback method plus Listener."

Bug: 126392011
Change-Id: I9a883765541ae853c4eb3bd52c9d93a7af16e661
Test: Manual build