AAPT2: Fix issue with parsing escape sequences when the parser only gives us part at a time

Change-Id: I671955f350b57577a991dd88f6a7319ec0b36c07
diff --git a/tools/aapt2/StringPiece.h b/tools/aapt2/StringPiece.h
index e2a1597..8cbdeae 100644
--- a/tools/aapt2/StringPiece.h
+++ b/tools/aapt2/StringPiece.h
@@ -229,4 +229,9 @@
 
 } // namespace aapt
 
+inline ::std::ostream& operator<<(::std::ostream& out, const std::u16string& str) {
+    android::String8 utf8(str.data(), str.size());
+    return out.write(utf8.string(), utf8.size());
+}
+
 #endif // AAPT_STRING_PIECE_H