Don't spam MSG_CHECK_PENDING_COLD_START_VALIDITY

I see many hundreds of these messages queued up during boot. We only
need to have at most one of these messages oustanding. Therefore we
only queue one MSG_CHECK_PENDING_COLD_START_VALIDITY at a time.

Bug: 308488707
Test: Boot phone and use it, check that message counts are
Test: back to normal.
Change-Id: I0dbc0ff08edb6db8983765e1119b372594847eb9