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/AppInfo.h b/tools/aapt2/AppInfo.h
index 51d8ca6..1d39b72 100644
--- a/tools/aapt2/AppInfo.h
+++ b/tools/aapt2/AppInfo.h
@@ -31,12 +31,12 @@
     /**
      * App's package name.
      */
-    std::u16string package;
+    std::string package;
 
     /**
      * The App's minimum SDK version.
      */
-    Maybe<std::u16string> minSdkVersion;
+    Maybe<std::string> minSdkVersion;
 };
 
 } // namespace aapt