Non-transient non-serializable instance field in WatermarkHold

[FindBugs SE_BAD_FIELD](https://github.com/apache/incubator-beam/blob/58a029a06aea1030279e5da8f9fa3114f456c1db/sdks/java/build-tools/src/main/resources/beam/findbugs-filter.xml#L477): Non-transient non-serializable instance field in serializable class

Applies to: [WatermarkHold.timerInternals](https://github.com/apache/incubator-beam/blob/58a029a06aea1030279e5da8f9fa3114f456c1db/runners/core-java/src/main/java/org/apache/beam/sdk/util/WatermarkHold.java#L81).

This is a good starter bug. When fixing, please remove the corresponding entries from [findbugs-filter.xml](https://github.com/apache/incubator-beam/blob/master/sdks/java/build-tools/src/main/resources/beam/findbugs-filter.xml) and verify the build passes.

Imported from Jira [BEAM-420](https://issues.apache.org/jira/browse/BEAM-420). Original Jira may contain additional context.
Reported by: swegner.
This issue has child subcomponents which were not migrated over. See the original Jira for more information.

Start in runners/core-java/src/main/java/org/apache/beam/sdk/util/WatermarkHold.java and inspect the timerInternals field in the WatermarkHold class. Then review the corresponding entries in sdks/java/build-tools/src/main/resources/beam/findbugs-filter.xml. The work is complete when the issue is addressed, those filter entries are removed, and the build passes.

Written by the indexing model from the issue text.