Camera2: Check for sequence complete sooner.

It's possible, especially with legacy mode, for a last frame for a
sequence to have already arrived when a repeating sequence is stopped.

In that case, need to check for sequence complete as soon as the
requestId, framenumber pair is added to the pending queue of completions.

Bug: 17428716
Change-Id: Ie5c6ea5a43639c812b8441da326c6303e150fce0