BurstMemoryCache serialization cleanup

This CL offers some prerequisite cleanup for the following two
subsequent changes:
(1) Handle case where Burst FMQ packet exceeds FMQ buffer size
(2) Create validation tests for FMQ serialized message format

It does this by making the serialization and deserialization functions
more modular and their usage clearer.

This CL additionally changes the worker from std::future to std::thread.
This is because some implementations of std::future use a
ThreadPool/WorkPool, which can cause unintended deadlocks if not handled
carefully. std::thread does not have those same concerns.

Bug: 129779280
Bug: 129157135
Test: mma
Test: atest NeuralNetworksTest_static
Test: atest VtsHalNeuralnetworksV1_0TargetTest (with sample-all)
Test: atest VtsHalNeuralnetworksV1_1TargetTest (with sample-all)
Test: atest VtsHalNeuralnetworksV1_2TargetTest (with sample-all)
Change-Id: Ia0842bc62c37c676c4faf83fc177198adac1f832
Merged-In: Ia0842bc62c37c676c4faf83fc177198adac1f832
(cherry picked from commit 1a57745f547572ed4fcfd77570e109b1c2288d5a)