Split UTF functions from String8/16

Split out all the UTF-8/16/32 handling code from String8/16 to its own
file to allow better reuse of code.

Change-Id: If9ce63920edc75472c38da4adce0d13cda9ad2f7
diff --git a/libs/utils/tests/Android.mk b/libs/utils/tests/Android.mk
index 00077ee..72d4876 100644
--- a/libs/utils/tests/Android.mk
+++ b/libs/utils/tests/Android.mk
@@ -8,7 +8,8 @@
 test_src_files := \
 	ObbFile_test.cpp \
 	Looper_test.cpp \
-	String8_test.cpp
+	String8_test.cpp \
+	Unicode_test.cpp
 
 shared_libraries := \
 	libz \