libbrillo: factor out Properties interface setup

This code is copy-pasted in the async and sync init functions.
Factor it out into its own function for brevity.

BUG=None
TEST=unit tests

Change-Id: I2c3c4f18df708065bebf77792a9d2d4c85022de4
Reviewed-on: https://chromium-review.googlesource.com/512984
Commit-Ready: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
2 files changed