Light refactors to packet wakeup events.

This patch uses the recently added MacAddress class in WakeupEvent and
WakeupStats for replacing the byte array representation of dest mac
addresses in packet wakeup events.

Bug: 28806131
Test: runtest frameworks-net
Merged-In: I5a3c76498a4b720f0d9308a65b5dd4b32377d0d1

(partial cherry pick from 884970e0)

Change-Id: I878d51d4cc53c6d9a2cc3c55faa15b86156afbd7