Add parameter for telling apps to start as services

This will allow apps to be conditionally started as
services. The enable_headless_music_mode flag is also added.

Bug: internal b/64904346
Test: Set the enable_headless_music_mode flag and test that GPM and YTM start as services.
Change-Id: Ibbdf4f1874438288659bd4988567a5826986a538
Reviewed-on: https://chromium-review.googlesource.com/654298


Commit-Queue: Aidan Wolter <awolter@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501357}