Add unit tests for AVB service

This patch adds unit tests that test request handling for the AVB app.
The tests are to be built and run on the host, not on the device.

unittest - unit tests that construct requests and validate responses
secure_storage_fake - fake implementation of SecureStorageInterface

Bug: 33051168
Test: "$ make unittest" builds, "$ ./unittest" shows that all tests pass
Change-Id: I8181af8881ec15d1d9d67835e2f7245d6ed55b14
3 files changed