aidl: Options - avoid calling exit(3)

The fuzzer considers this a fatal error.

Bug: 195473218
Test: aidl_unittests + the following commands:
    # in a directory without a file named
    # 'alskdjflsakdjflkasjdflkajdslfkajsdf', this command fails:
    aidl alskdjflsakdjflkasjdflkajdslfkajsdf

    # this command returns a successful result, and nothing else is done
    aidl --help

Change-Id: I9c0509dae8c4516d8ca57b7a203b51acb5848725
diff --git a/options.h b/options.h
index ebbd004..9f645a8 100644
--- a/options.h
+++ b/options.h
@@ -77,7 +77,7 @@
  public:
   enum class Language { UNSPECIFIED, JAVA, CPP, NDK, RUST };
 
-  enum class Task { UNSPECIFIED, COMPILE, PREPROCESS, DUMP_API, CHECK_API, DUMP_MAPPINGS };
+  enum class Task { UNSPECIFIED, HELP, COMPILE, PREPROCESS, DUMP_API, CHECK_API, DUMP_MAPPINGS };
 
   enum class CheckApiLevel { COMPATIBLE, EQUAL };