AAPT2: Remove usage of u16string

For legacy reasons, we kept around the use of UTF-16 internally
in AAPT2. We don't need this and this CL removes all instances of
std::u16string and StringPiece16. The only places still needed
are when interacting with the ResTable APIs that only operate in
UTF16.

Change-Id: I492475b84bb9014fa13bf992cff447ee7a5fe588
diff --git a/tools/aapt2/xml/XmlDom_test.cpp b/tools/aapt2/xml/XmlDom_test.cpp
index 431ee2c..1909f75 100644
--- a/tools/aapt2/xml/XmlDom_test.cpp
+++ b/tools/aapt2/xml/XmlDom_test.cpp
@@ -43,8 +43,8 @@
 
     xml::Namespace* ns = xml::nodeCast<xml::Namespace>(doc->root.get());
     ASSERT_NE(ns, nullptr);
-    EXPECT_EQ(ns->namespaceUri, u"http://schemas.android.com/apk/res/android");
-    EXPECT_EQ(ns->namespacePrefix, u"android");
+    EXPECT_EQ(ns->namespaceUri, xml::kSchemaAndroid);
+    EXPECT_EQ(ns->namespacePrefix, "android");
 }
 
 } // namespace aapt