only expecting one cleanup method per class

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### Description of the problem / feature request:

I was instructed to report this and post this information

```
error-prone version: 2.4.0
BugPattern: UnbalancedListener
Stack Trace:
java.lang.RuntimeException: only expecting one cleanup method per class
at com.uber.errorprone.checker.leaks.UnbalancedListener$CallScanner.visitMethod(UnbalancedListener.java:270)
at com.uber.errorprone.checker.leaks.UnbalancedListener$CallScanner.visitMethod(UnbalancedListener.java:173)
at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:898)
at com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.sun.source.util.TreeScanner.scan(TreeScanner.java:105)
at com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:113)
at com.sun.source.util.TreeScanner.visitClass(TreeScanner.java:187)
at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:808)
at com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:90)
at com.sun.source.util.TreeScanner.scan(TreeScanner.java:105)
at com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:113)
at com.sun.source.util.TreeScanner.visitClass(TreeScanner.java:187)
at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:808)
at com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:56)
at com.uber.errorprone.checker.leaks.UnbalancedListener.matchClass(UnbalancedListener.java:141)
at com.google.errorprone.scanner.ErrorProneScanner.processMatchers(ErrorProneScanner.java:451)
at com.google.errorprone.scanner.ErrorProneScanner.visitClass(ErrorProneScanner.java:549)
at com.google.errorprone.scanner.ErrorProneScanner.visitClass(ErrorProneScanner.java:152)
at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:808)
at com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
at com.sun.source.util.TreeScanner.scan(TreeScanner.java:105)
at com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:113)
at com.sun.source.util.TreeScanner.visitCompilationUnit(TreeScanner.java:144)
at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:562)
at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:152)
at com.sun.tools.javac.tree.JCTree$JCCompilationUnit.accept(JCTree.java:591)
at com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:56)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:58)
at com.google.errorprone.scanner.ErrorProneScannerTransformer.apply(ErrorProneScannerTransformer.java:43)
at com.google.errorprone.ErrorProneAnalyzer.finished(ErrorProneAnalyzer.java:152)
at com.sun.tools.javac.api.MultiTaskListener.finished(MultiTaskListener.java:120)
at com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1404)
at com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1353)
at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:946)
at com.sun.tools.javac.api.JavacTaskImpl.lambda$doCall$0(JavacTaskImpl.java:100)
at com.sun.tools.javac.api.JavacTaskImpl.handleExceptions(JavacTaskImpl.java:142)
at com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:96)
at com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:90)
at com.facebook.buck.jvm.java.plugin.adapter.JavacTaskWrapper.call(JavacTaskWrapper.java:100)
at com.facebook.buck.jvm.java.plugin.adapter.BuckJavacTask.call(BuckJavacTask.java:52)
at com.facebook.buck.jvm.java.plugin.adapter.BuckJavacTaskProxyImpl.call(BuckJavacTaskProxyImpl.java:134)
at com.facebook.buck.jvm.java.Jsr199JavacInvocation$CompilerWorker.lambda$startCompiler$2(Jsr199JavacInvocation.java:424)
at com.google.common.util.concurrent.TrustedListenableFutureTask$TrustedFutureInterruptibleTask.runInterruptibly(TrustedListenableFutureTask.java:127)
at com.google.common.util.concurrent.InterruptibleTask.run(InterruptibleTask.java:57)
at com.google.common.util.concurrent.TrustedListenableFutureTask.run(TrustedListenableFutureTask.java:80)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
```