Separate H4 packet parsing from data retrieval.

We are in the process of migrating Rootcanal to support various
architectures as well as different data channels.

For this reason we separate the H4 packetizer into a packet parser and
a packetizer that reads/writes packets from a file descriptor.

This will make it possible to use the H4 protcol with different data
channels that are not based on file descriptors (Qemu/gRPC/Windows)

Test: Additional set of unit tests to validate the parser.
Change-Id: I3a8f24197fed953b86d228f0bc2e793249323a62