Make RecordedTransaction format exensible

This patch changes the file format for RecordedTransactions towards
stable and extensibility. Warnings about stability are retained so that
the TransactionHeader may be more easily changed to serve our needs for
a "version 1".

Test: `atest binderUnitTest`

Change-Id: If8d63304815a227a8ee2bb8f7bfaa1ab33bd6e0f