CommandProcessor: add setters for TimestampHeader

- Add setters for the fields of TimestampHeader,
  so that we can use chaining to initialize a
  TimestampHeader.
- Update CommandProcessor, to make use of this
  ability.
- Move TimestampHeader out of command_processor.h,
  to reduce clutter in the header file.

Bug: 32327379
Test: ./runtests.sh (on angler)
Change-Id: I3ffb29d47160d533947012d9ede1cadd4d954935
2 files changed