Clean up options parsing logic a little bit

We're about to add support for asking for C++ output.
Make this a little less tedious and flesh out the unittests a little.

Bug: 23599341
Test: unittests

Change-Id: Ie08414db43b48ae1a90640260b0badf4c4eb60f8
diff --git a/options.h b/options.h
index 018345e..c8ac8a8 100644
--- a/options.h
+++ b/options.h
@@ -31,8 +31,8 @@
 class Options {
  public:
   enum {
-      COMPILE_AIDL,
-      PREPROCESS_AIDL
+      COMPILE_AIDL_TO_JAVA,
+      PREPROCESS_AIDL,
   };
 
   ~Options() = default;
@@ -43,7 +43,7 @@
   static std::unique_ptr<Options> ParseOptions(int argc,
                                                const char* const* argv);
 
-  int task{COMPILE_AIDL};
+  int task{COMPILE_AIDL_TO_JAVA};
   bool fail_on_parcelable_{false};
   std::vector<std::string> import_paths_;
   std::vector<std::string> preprocessed_files_;