Watermark hold for timer output timestamp is not computed correctly

With the introduction of timer output timestamp, a new watermark hold had been added to the Flink Runner. The watermark computation works on the keyed state backend which computes a key-scoped watermark hold and not the desired operator-wide watermark hold.

Computation: https://github.com/apache/beam/blob/b564239081e9351c56fb0e7d263495b95dd3f8f3/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/DoFnOperator.java#L1140

Key-scoped state: https://github.com/apache/beam/blob/b564239081e9351c56fb0e7d263495b95dd3f8f3/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/DoFnOperator.java#L1130

We need to change this to operate on all keys. This has to be done before fixing BEAM-9566.