ueventd: parse seqnum field of a uevent

Seqnum field is a unique ID of a uevent. This change lets
uevent_listener to parse it, which is useful to identify uevents with
the same paths.

Bug: 400592897
Test: Cuttlefish boots and uevent is parsed correctly
Change-Id: I0d4e3e75320440c14895de52a824617a9a536356