Support playNow on queued SLEEP transits

When a transition is queued, we usually post the onCollectStarted
callback to avoid ordering issues w/ the prior transition. This
means that anything in the handler queue will execute before
onCollectStarted. Again, this is desired normally and it means
those actions will be collected in the new transition.

However, SLEEP transitions are currently treated as empty
sentinels (at-least today). While there's nothing wrong with
collecting changes in SLEEP transitions, we still have some
transitions (eg. around keyguard) that can race with it, so
to reduce those races short-term, this adds a check for SLEEP
when dequeuing and start that immediately vs. posting.

Bug: 304713150
Test: Existing tests pass.
Change-Id: Id234414c220367a2e971fa29cf03ba631c7564f1