Limit max running user to 2 for car

- In car, headless user 0 will be always running. This change will
  limit running users into headless user 0 and one foreground user.
- This forces stopping previous foreground user when user switching
  happens. That will clean up all background Servies.

Bug: 128038498
Test: Check user state and Service clean up after user switching
      adb shell dumpsys activity|grep ServiceRecord
      adb shell dumpsys user

Change-Id: Ia15a1a79bd2ab320d197f0cf514e2ff961099579
1 file changed