Do not call setQuietModeEnabled from the main thread

Call into startUserInBackgroundWithListener->onBeforeStartUser sometimes
can be blocked for a long time if there is an ongoing dexopt operation.

Instead call it from bg thread.

Test: manual
Bug: 72692931
Change-Id: I872c3a2baf6e5ff95aab186f7a8008afec7533d6