test_vendor: Add HciPacket and Iterator classes

HciPacket is the abstract base class for all packet types.

Iterator is the custom iterator class that all packet types will use
to iterate through. The iterators in this class are bounded from
[start_of_packet, length_of_packet]. This prevents incrementing the iterator
passed the end sentinel.

Test: Passes all of the tests in test/iterator_test.cc
Change-Id: If9fec9ae32d9a23575ad3e9219b8249d164bc0d5