ueventd: introduce UeventDependencyGraph class

Uevnt messages can be handled in parallel in a robust way only if we
parallelize events that have no dependency on others.
UeventDependencyGraph class manages the dependencies among Uevents, and
lets callers to take a Uevent which has no dependency on preceding
pending events.

Bug: 400592897
Test: unit tests pass
Change-Id: Ide51058480f6a7e3f630b7ac974d7090b922a2d6