Allow more than two transitions per track

Before this CL we would mark an incoming transition as 'sync' if we'd
find two different already playing transitions - even if both the
playing transitions were running on the same track.

With this CL we allow more than two transitions to play on the same
track.

Bug: 402417624
Flag: EXEMPT - bugfix
Test: manual
Change-Id: I3c3efb5f7f3db2bab692a3bfecba615b0d30ddfd