8358588: ThreadSnapshot.ThreadLock should be static nested class

Reviewed-by: alanb, sspitsyn, amenkov

Author: shipilev

src/java.base/share/classes/jdk/internal/vm/ThreadSnapshot.java

@@ -175,7 +175,7 @@ private enum BlockerLockType {
     /**
      * Represents a locking operation of a thread at a specific stack depth.
      */
-    private class ThreadLock {
+    private static class ThreadLock {
         private static final OwnedLockType[] lockTypeValues = OwnedLockType.values(); // cache
 
         // set by the VM