Send system user broadcasts in headless system user mode.

In headless system user mode, USER_STARTING, USER_STARTED, and
USER_SWITCHED broadcasts were not sent for the system user.
It had caused issues for other parts of the system which are expecting
such broadcast messages. For example, VpnManagerService expects
USER_STARTED event for system user but was not receiving it.

This change will only affect headless system user mode. No behavior
changes for phones.

Bug: 263439429
Bug: 242195409

Test: atest com.android.cts.devicepolicy.MixedDeviceOwnerTest

Change-Id: I4a101418c10a2c959c2bfae01b95863aebd521e8