apmanager: remove the AP service when its owner disappeared

This prevents AP services from becoming orphans when their respective
owner crashed.

BUG=brillo:398
TEST=Run apmanager_CheckAPProcesses and privetd_BasicDBusAPI test
Manual Test:
1. Start an AP service with privetd
2. Simulate privetd crash using SIGKILL signal
3. Verify the AP service is terminated

Change-Id: I191a3651dd9cc8cc0a3607cea60448417310f991
Reviewed-on: https://chromium-review.googlesource.com/254608
Reviewed-by: Zeping Qiu <zqiu@chromium.org>
Commit-Queue: Zeping Qiu <zqiu@chromium.org>
Trybot-Ready: Zeping Qiu <zqiu@chromium.org>
Tested-by: Zeping Qiu <zqiu@chromium.org>
5 files changed