Add sequence number to events in inputflinger

This adds a sequece number field to NotifyArgs and EventEntry in order to
uniquely track individual events.

The sequence number for the events is added in InputReader, where the next
sequence number is an increment of the previous event's sequence number.
Synthesized events have a sequence number of 0.

Bug: 119312685
Test: inputflinger_tests
Change-Id: I1690979a5aa019e99ab89f5f7279a1b42068e658