DO NOT MERGE - Enable blacklist for headless system user

- Get all packages for system user
- Reuse split system user's logic to blacklist packages when headless system
user is enabled
- Add new method in UserManager to get headless sytem user mode (same as
in master)

Bug: 145626101
Test: edit device's sysconfig file, use tag: system-user-blacklisted-app to blacklist app
make services && adb sync system && adb reboot
cts test

Change-Id: I98d1bc33e7dd59ffa3ac6426f95af708671138da
(cherry picked from commit 9c4cf4945d57d45e429bc7d33bf6ffa68eb0838c)