Queue startTransition calls made while already collecting

Shell Transitions only supports 1 transition collecting at a time.
This limitation comes from BLASTSyncEngine. However, because Shell
can call startTransition at any time (and doesn't have a way to
know if WM is already "collecting" for a transition), we can't
just crash in this case. We also can't just use the existing
transition because it has a different "client" expecting results.

So, the best we can do is queue them up. Fortunately, the
startTransition API Shell uses is designed such that all the
relevant operations are in one WCT, so we can maintain server-side
correctness.

This CL adds a queueing mechanism for these situations. In practice,
this should actually never be needed; however, it is *technically*
possible and automated tests are more likely to hit it.

Bug: 183993924
Test: pass existing
Change-Id: I255fb336cb30c7d1e1b82188a31f9c693b9e138c