Use the icu:: namespace for icu4c API.

The icu guys recommend working this way, though it's not the default.
Internally Google makes this the default, and I've switched libcore
over.

Bug: 15350832
Change-Id: I038e12bea58dd089a46d0e681b5ab76cfc19d7b9
diff --git a/core/jni/Android.mk b/core/jni/Android.mk
index 20ece24..45dce2c 100644
--- a/core/jni/Android.mk
+++ b/core/jni/Android.mk
@@ -22,6 +22,8 @@
 
 LOCAL_CFLAGS += -DGL_GLEXT_PROTOTYPES -DEGL_EGLEXT_PROTOTYPES
 
+LOCAL_CFLAGS += -DU_USING_ICU_NAMESPACE=0
+
 LOCAL_SRC_FILES:= \
     AndroidRuntime.cpp \
     com_android_internal_content_NativeLibraryHelper.cpp \