Don't unset syncId in Transition.

mSyncId was being used to check if "collecting" but then it would
be -1 for logging and other purposes. This required mDebugId.

This is a little silly though, so instead use mState to track
"collecting" so that we can leave mSyncId unchanged -- this also
allows logic based on the sync-id after it has started playing.

Bug: 233625646
Test: no material change, tests still pass
Change-Id: I00a0200c476f4f31c9b81c26d65cc4531d91e24d