audio: Enable more compile time checks in the default impl

Enable "-Wall, Wextra, Werror, Wthread-safety",
fix discovered issues.

Bug: 205884982
Test: m
Change-Id: I0a8d3095dd24dbb3bc7cf6569c1f71945cd55168
diff --git a/audio/aidl/default/Android.bp b/audio/aidl/default/Android.bp
index c9edae0..31ed044 100644
--- a/audio/aidl/default/Android.bp
+++ b/audio/aidl/default/Android.bp
@@ -92,6 +92,12 @@
         "audio_policy_configuration_aidl_default",
         "audio_policy_engine_configuration_aidl_default",
     ],
+    cflags: [
+        "-Wall",
+        "-Wextra",
+        "-Werror",
+        "-Wthread-safety",
+    ],
 }
 
 cc_binary {