libgui: Enable -Weverything and -Werror

Enables -Weverything and -Werror, with just a few exceptions for
warnings we can't (or shouldn't need to) work around.

Change-Id: I034abec27bf4020d84af60d7acc1939c59986dd6
diff --git a/libs/gui/BufferSlot.cpp b/libs/gui/BufferSlot.cpp
index b8877fe..01595de 100644
--- a/libs/gui/BufferSlot.cpp
+++ b/libs/gui/BufferSlot.cpp
@@ -24,8 +24,8 @@
         case BufferSlot::QUEUED: return "QUEUED";
         case BufferSlot::FREE: return "FREE";
         case BufferSlot::ACQUIRED: return "ACQUIRED";
-        default: return "Unknown";
     }
+    return "Unknown";
 }
 
 } // namespace android