Send user starting broadcast for non-automotive devices in HSUM.

On automotive HSUM (Headless System User Mode) devices, user switch is
called by CarService as user 0, thus ActivityManagerService#systemReady
can send USER_STARTED and USER_STARTING broacasts for the system user.
However on non-automotive HSUM devices, such as tablets, the broadcasts
were not sent because user switch is called by the system_server, and
ActivityManagerService#systemReady runs as user 10.
This change updates  UserController#startUserInternal() to also send
broadcasts for the system user starting in HSUM.

This change will only affect headless system user mode, and should not
affect phones.

Bug: 266594620

Test: Tested on 3 targets: automotive HSUM, non-automotive HSUM, phone
Test: Checked the EVENT LOG in the bugreport for 'uc_send_user_broadcast'
Test: Checked if VPN is configured for the system user:
      $ adb shell dumpsys vpn_management
Test: atest UserControllerTest ActivityManagerServiceTest

Change-Id: I1c81e1ea07f89eea7523d2c152db6616719c5641