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/diagnostics_unittest.cpp b/diagnostics_unittest.cpp
index 8c13df4..0066b9f 100644
--- a/diagnostics_unittest.cpp
+++ b/diagnostics_unittest.cpp
@@ -218,12 +218,12 @@
                "interface IFoo{}"}});
 }
 
-TEST_F(DiagnosticsTest, AllowRedundantImports) {
-  expect_diagnostics = {};
+TEST_F(DiagnosticsTest, RedundantImports) {
+  expect_diagnostics = {DiagnosticID::unique_import};
   ParseFiles({{"p/IFoo.aidl",
                "package p;\n"
                "import q.IBar;\n"
-               "import q.IBar;\n"  // ugly, but okay
+               "import q.IBar;\n"
                "interface IFoo{}"},
               {"q/IBar.aidl", "package q; interface IBar{}"}});
 }