Stop user faster and clear stale broadcasts

Moved several USER_STOPPING registered receivers to listen
to USER_STOPPED, since they don't need to be blocking the
shutdown of the user.

Clear all stale broadcasts when stopping a user, so that we
don't unnecessarily start up processes and deliver stale
broadcasts. This was causing code to run when the user was
already stopped and resulted in crashes when other providers
and services couldn't be started anymore. Hopefully this fixes
many of those races.

Bug: 28371487
Change-Id: Ic35a7a23fa8fe009a53f8bf7545d4dad5fa34134