Enable -Wall -Wextra for libcore and cleanup all but one warning

Change-Id: Ied76662c470ba878cec61189acf29f5cbbd4ccd4
diff --git a/NativeCode.mk b/NativeCode.mk
index 41ca7cb..d448877 100644
--- a/NativeCode.mk
+++ b/NativeCode.mk
@@ -85,6 +85,8 @@
 
 include $(CLEAR_VARS)
 
+LOCAL_CFLAGS += -Wall -Wextra
+
 ifeq ($(TARGET_ARCH),arm)
 # Ignore "note: the mangling of 'va_list' has changed in GCC 4.4"
 LOCAL_CFLAGS += -Wno-psabi