Fix EntitlementManager issues

1. Add TETHERING_ETHERNET to vaild downstream type. So starting
   ethernet tethering will do entitlement check as well.
2. Ignore request with invalid downstream type on
   handleRequestLatestTetheringEntitlementValue()

Bug: 152828758
Bug: 152828142
Test: atests TetheringTests CtsTetheringTest

Change-Id: Id0cb59cc4681f5ffbde7be54de05a05e46f0ffb8