Send admin enabled broadcast in foreground queue

Currently admin enabled broadcast is sent as a background broadcast
upon ACTION_USER_STARTED. This can lead to delays on managed secondary
users. During that period the admin has no chance to apply any
policies, effectively leaving the user unmanaged. Sending it as a
foreground broadcast should speed up that process.

Using a foreground broadcast on ACTION_USER_STARTED can cause a race
condition where the user is not unlocked yet and therefore the
broadcast is omitted (unless the receiver is direct boot aware).
Sending the broadcast upon ACTION_USER_UNLOCKED fixes that issue.

Bug: 64382185
Test: manual
Change-Id: I3a89ba2e64cd6013723699cc1211b0144db254a6