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/main.cpp b/main.cpp
index 7aba3fb..7fc4193 100644
--- a/main.cpp
+++ b/main.cpp
@@ -29,7 +29,7 @@
   }
 
   switch (options->task) {
-    case Options::COMPILE_AIDL:
+    case Options::COMPILE_AIDL_TO_JAVA:
       return android::aidl::compile_aidl(*options);
     case Options::PREPROCESS_AIDL:
       return android::aidl::preprocess_aidl(*options);