CRAS: add GetDefaultOutputBufferSize dbus API

This is a reland of http://crosreview.com/605340. The differences
are:
- CRAS_SERVER_STATE_VERSION is not changed. The new variable is
  added at the end so it is backward compatible.
- Name is changed from GetMinOutputBufferSize to
  GetDefaultOutputBufferSize.

BUG=chromium:749345
TEST=Add a board.ini in cras-config and see chrome read the
     minimum output buffer size from the config.

Change-Id: I240bb39a1fe15bf940e25f278244e7283cbe03fe
Reviewed-on: https://chromium-review.googlesource.com/612926
Commit-Ready: Wu-Cheng Li <wuchengli@chromium.org>
Tested-by: Wu-Cheng Li <wuchengli@chromium.org>
Reviewed-by: Cheng-Yi Chiang <cychiang@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
9 files changed