Parse postinstall program progress updates.

In Android postinstall is expected to take a long time in common cases.
This patch allows the postinstall program to report back to the updater
a progress indication, which will then be forwarded to all the clients
listening. These progress updates are part of the FINALIZING status.

Bug: 27880754
TEST=Added unittests. Deployed an update to an edison-eng and post-install reported progress back with the postinstall_example.

(cherry picked from commit 0d29854c)

Change-Id: I003e996bd5a9acccfc83012345351174d269107c