Lint: Redundant imports

Following the Java language, redundant imports are allowed and just ignored.
But now the linter detects and complains about it as violating "unique-import".

Bug: 215566892
Test: aidl_unittests
Change-Id: Ibe575fa73f8e47c8646f1807f7c9df94f2a9ce7a
diff --git a/parser.h b/parser.h
index e497912..074d6d5 100644
--- a/parser.h
+++ b/parser.h
@@ -92,7 +92,7 @@
   const std::string& Package() const { return package_; }
 
   void MakeDocument(const AidlLocation& location, const Comments& comments,
-                    std::set<std::string> imports,
+                    std::vector<std::string> imports,
                     std::vector<std::unique_ptr<AidlDefinedType>> defined_types);
 
  private: