MessageBuffer: add Rewind()

Provide the ability to rewind a MessageBuffer, and
appropriate tests for that ability. The purpose of
Rewind() is to allow a MessageBuffer to be dumped
multiple times.

Bug: 31653003
Test: ./runtests.sh (on bullhead)
Change-Id: Ic3cc1766bbd3068d96584a938c181c68c62ab55d
3 files changed