Make libutils symbols protected

This change is a workaround for apps linking
libutils statically and dynamically which causes
them to crash for newer version of Android.

Bug: http://b/27313399
Change-Id: I47ac4146041b6eeef03cb605ea436719d552ec8f
diff --git a/libutils/Android.mk b/libutils/Android.mk
index 8c4fd15..3584b6a 100644
--- a/libutils/Android.mk
+++ b/libutils/Android.mk
@@ -72,7 +72,7 @@
 ifeq ($(TARGET_ARCH),mips)
 LOCAL_CFLAGS += -DALIGN_DOUBLE
 endif
-LOCAL_CFLAGS += -Werror
+LOCAL_CFLAGS += -Werror -fvisibility=protected
 
 LOCAL_STATIC_LIBRARIES := \
 	libcutils \