StateTags$CombiningValueStateTag inherits equals from super class but without incorporating its state

[FindBugs EQ_DOESNT_OVERRIDE_EQUALS](https://github.com/apache/incubator-beam/blob/58a029a06aea1030279e5da8f9fa3114f456c1db/sdks/java/build-tools/src/main/resources/beam/findbugs-filter.xml#L555): Class doesn't override equals in superclass

Applies to: [StateTags$CombiningValueStateTag](https://github.com/apache/incubator-beam/blob/58a029a06aea1030279e5da8f9fa3114f456c1db/sdks/java/core/src/main/java/org/apache/beam/sdk/util/state/StateTags.java#L344). CombiningValueStateTag inherits its equals implementation from its parent class, KeyedCombiningValueStateTag. CombiningValueStateTag has additional fields which are not included in the equality implementation, which may be incorrect.

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-421](https://issues.apache.org/jira/browse/BEAM-421). Original Jira may contain additional context.
Reported by: swegner.

Start in sdks/java/core/src/main/java/org/apache/beam/sdk/util/state/StateTags.java at StateTags$CombiningValueStateTag and compare its inherited equality with its additional fields. Then inspect the corresponding entries in sdks/java/build-tools/src/main/resources/beam/findbugs-filter.xml. Done means the equality issue is addressed, the filter entries are removed, and the Java build passes.

Written by the indexing model from the issue text.