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/Android.mk b/libs/utils/Android.mk
index eb75ed8..05a9674 100644
--- a/libs/utils/Android.mk
+++ b/libs/utils/Android.mk
@@ -41,6 +41,7 @@
 	TextOutput.cpp \
 	Threads.cpp \
 	Timers.cpp \
+	Unicode.cpp \
 	VectorImpl.cpp \
 	ZipFileCRO.cpp \
 	ZipFileRO.cpp \