Use BaseNetworkObserver in Tethering.

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 f4e90eac]

Bug: 9180552
Change-Id: I74859b0d77951005651aaaa418185857e40eeedb