Fix findbugs warnings in hadoop-yarn-project on branch-2.10

$ find . -name findbugsXml.xml | xargs -n 1 /opt/findbugs-3.0.1/bin/convertXmlToText -longBugCodes
M P WMI_WRONG_MAP_ITERATOR WMI: org.apache.hadoop.yarn.state.StateMachineFactory.generateStateGraph(String) makes inefficient use of keySet iterator instead of entrySet iterator  At StateMachineFactory.java:[line 505]
M P BX_UNBOXING_IMMEDIATELY_REBOXED Bx: Boxed value is unboxed and then immediately reboxed in org.apache.hadoop.yarn.server.timelineservice.storage.common.ColumnRWHelper.readResultsWithTimestamps(Result, byte[], byte[], KeyConverter, ValueConverter, boolean)  At ColumnRWHelper.java:[line 335]
M D UC_USELESS_OBJECT UC: Useless object stored in variable removedNullContainers of method org.apache.hadoop.yarn.server.nodemanager.NodeStatusUpdaterImpl.removeOrTrackCompletedContainersFromContext(List)  At NodeStatusUpdaterImpl.java:[line 664]
M P WMI_WRONG_MAP_ITERATOR WMI: org.apache.hadoop.yarn.server.nodemanager.NodeStatusUpdaterImpl.removeVeryOldStoppedContainersFromCache() makes inefficient use of keySet iterator instead of entrySet iterator  At NodeStatusUpdaterImpl.java:[line 741]
M P WMI_WRONG_MAP_ITERATOR WMI: org.apache.hadoop.yarn.server.nodemanager.containermanager.localizer.ContainerLocalizer.createStatus() makes inefficient use of keySet iterator instead of entrySet iterator  At ContainerLocalizer.java:[line 359]
M V MS_MUTABLE_COLLECTION_PKGPROTECT MS: org.apache.hadoop.yarn.server.nodemanager.containermanager.monitor.ContainerMetrics.usageMetrics is a mutable collection which should be package protected  At ContainerMetrics.java:[line 134]