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/Flags.cpp b/tools/aapt2/Flags.cpp
index 666e8a8e..2033a4c 100644
--- a/tools/aapt2/Flags.cpp
+++ b/tools/aapt2/Flags.cpp
@@ -101,7 +101,7 @@
         // Split the description by newlines and write out the argument (which is empty after
         // the first line) followed by the description line. This will make sure that multiline
         // descriptions are still right justified and aligned.
-        for (StringPiece line : util::tokenize<char>(flag.description, '\n')) {
+        for (StringPiece line : util::tokenize(flag.description, '\n')) {
             *out << " " << std::setw(kWidth) << std::left << argLine << line << "\n";
             argLine = " ";
         }