Use fine-grained enums to represent MAC_ADDRESS_CHANGED failure.

Currently as long as the neighbor is reachable but the default gateway's
mac address has changed, NUD_MAC_ADDRESS_CHANGED will be used as the
event type of neighbor loss. However, in fact we don't know the cause of
neighbor's mac changes, e.g. it happened after roaming or after confirm
or orgnaic probes. Replace NUD_MAC_ADDRESS_CHANGED with new constants.

Bug: 162944199
Test: atest NetworkStackTests NetworkStackIntegrationTests
Change-Id: Ica7f87051ef9e0718e5bda54c33d492729382e78