make all libsurfaceflinger and libsensorservice symbols's visibility hidden

we only export the main entry-point.
this saves about 150KB.

Change-Id: I55eb2b6705386fdfa43860deb62f9cdd2a0982aa
diff --git a/services/surfaceflinger/Android.mk b/services/surfaceflinger/Android.mk
index ec296d3..71b235a 100644
--- a/services/surfaceflinger/Android.mk
+++ b/services/surfaceflinger/Android.mk
@@ -41,6 +41,8 @@
   LOCAL_CFLAGS += -DNUM_FRAMEBUFFER_SURFACE_BUFFERS=$(NUM_FRAMEBUFFER_SURFACE_BUFFERS)
 endif
 
+LOCAL_CFLAGS += -fvisibility=hidden
+
 LOCAL_SHARED_LIBRARIES := \
 	libcutils \
 	liblog \