Remove easy uses of //aos/mutex

I was hoping to get rid of it completely as one of the nontrivial users
of AOS_LOG, but need to write a compatible condition variable first.

Change-Id: I86c1c4084882bf789baabd0255e115d821154830
diff --git a/aos/testing/BUILD b/aos/testing/BUILD
index e70b5a4..1fd5380 100644
--- a/aos/testing/BUILD
+++ b/aos/testing/BUILD
@@ -26,7 +26,7 @@
         ":googletest",
         "//aos:thread_local",
         "//aos/logging:implementations",
-        "//aos/mutex",
+        "//aos/stl_mutex",
         "@com_google_absl//absl/base",
     ],
 )