Continuous updates should interrupt transitions (#21323)

Even when updates are sync by default.

Discovered this quirk while working on #21322. Previously, when sync
default updates are enabled, continuous updates are treated like
default updates. We implemented this by assigning DefaultLane to
continous updates. However, an unintended consequence of that approach
is that continuous updates would no longer interrupt transitions,
because default updates are not supposed to interrupt transitions.

To fix this, I changed the implementation to always assign separate
lanes for default and continuous updates. Then I entangle the
lanes together.