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/compile/Pseudolocalizer.h b/tools/aapt2/compile/Pseudolocalizer.h
index 8818c17..7db88de 100644
--- a/tools/aapt2/compile/Pseudolocalizer.h
+++ b/tools/aapt2/compile/Pseudolocalizer.h
@@ -29,10 +29,10 @@
 class PseudoMethodImpl {
 public:
     virtual ~PseudoMethodImpl() {}
-    virtual std::u16string start() { return {}; }
-    virtual std::u16string end() { return {}; }
-    virtual std::u16string text(const StringPiece16& text) = 0;
-    virtual std::u16string placeholder(const StringPiece16& text) = 0;
+    virtual std::string start() { return {}; }
+    virtual std::string end() { return {}; }
+    virtual std::string text(const StringPiece& text) = 0;
+    virtual std::string placeholder(const StringPiece& text) = 0;
 };
 
 class Pseudolocalizer {
@@ -45,9 +45,9 @@
 
     Pseudolocalizer(Method method);
     void setMethod(Method method);
-    std::u16string start() { return mImpl->start(); }
-    std::u16string end() { return mImpl->end(); }
-    std::u16string text(const StringPiece16& text);
+    std::string start() { return mImpl->start(); }
+    std::string end() { return mImpl->end(); }
+    std::string text(const StringPiece& text);
 private:
     std::unique_ptr<PseudoMethodImpl> mImpl;
     size_t mLastDepth;