Fix a concurrent modification exception when generating stubs
```
java.util.ConcurrentModificationException
at java.base/java.util.HashMap$HashIterator.nextNode(HashMap.java:1605)
at java.base/java.util.HashMap$ValueIterator.next(HashMap.java:1633)
at com.google.doclava.Stubs.writeStubsAndApi(Stubs.java:166)
at com.google.doclava.Doclava.start(Doclava.java:1731)
at com.google.doclava.Doclava.run(Doclava.java:1514)
at jdk.javadoc/jdk.javadoc.internal.tool.Start.parseAndExecute(Start.java:621)
at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:444)
at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:346)
at jdk.javadoc/jdk.javadoc.internal.tool.Main.execute(Main.java:57)
at jdk.javadoc/jdk.javadoc.internal.tool.Main.main(Main.java:46)
```
(cherry picked from https://android-review.googlesource.com/q/commit:e6e3b911ab40ad3ac0c45da6c8453ee72ef15aa7)
Merged-In: I7df9568d92764d8356cfe20001ff08b1b9d7e1ee
Change-Id: I7df9568d92764d8356cfe20001ff08b1b9d7e1ee
Bug: 278604691
1 file changed
