Tethering currently inherits from the AIDL interface INetworkManagementEventObserver, so it has to provide no-op implementations of all the interface's methods. Inherit from BaseNetworkObserver and get rid of the no-ops. [Cherry-pick of f4e90eac87bc31611ddd80c46fc924d35ce66c1c] Bug: 9180552 Change-Id: I74859b0d77951005651aaaa418185857e40eeedb