art: Try to fix parsed_options_test on the build server

(It works fine locally because environment variables are defined
differently)

Also some minor cleanup elsewhere pointed out in the previous code
review.

Change-Id: I80650d2be6ce467b838db85eb130a0020d7c1480
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index 4bb1741..549ef0e 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -762,7 +762,7 @@
 
   verify_ = runtime_options.GetOrDefault(Opt::Verify);
 
-  if (runtime_options.Exists(Opt::Interpret)) {
+  if (runtime_options.GetOrDefault(Opt::Interpret)) {
     GetInstrumentation()->ForceInterpretOnly();
   }