TriggerStateMachine.prefetchOnElement and other prefetch methods use incorrect state for subtriggers

prefetchOnElement takes a single StateAccessor corresponding to the root trigger. It uses that state accessor to perform prefetches for the subtrigger state. However that will use the root trigger namespace for the tags in the subtriggers. This makes prefetching ineffective, introducing possibly an additional round-trip once the data is actually read.

https://github.com/apache/beam/blob/68d6c8e6243b1d8f392840273f886276e2a8baff/runners/core-java/src/main/java/org/apache/beam/runners/core/triggers/TriggerStateMachine.java#L296