Move StringPiece to libandroidfw

libandroidfw needs to make use of StringPiece, so
move it to libandroidfw and update all code referencing
StringPiece in aapt2.

Test: make libandroidfw_tests libaapt2_tests
Change-Id: I68d7f0fc7c651b048d9d1f5e7971f10ef5349fa1
diff --git a/tools/aapt2/flatten/Archive.cpp b/tools/aapt2/flatten/Archive.cpp
index 47de0a3..5c96a4d 100644
--- a/tools/aapt2/flatten/Archive.cpp
+++ b/tools/aapt2/flatten/Archive.cpp
@@ -22,10 +22,12 @@
 #include <vector>
 
 #include "android-base/macros.h"
+#include "androidfw/StringPiece.h"
 #include "ziparchive/zip_writer.h"
 
 #include "util/Files.h"
-#include "util/StringPiece.h"
+
+using android::StringPiece;
 
 namespace aapt {
 
@@ -36,7 +38,7 @@
   DirectoryWriter() = default;
 
   bool Open(IDiagnostics* diag, const StringPiece& out_dir) {
-    dir_ = out_dir.ToString();
+    dir_ = out_dir.to_string();
     file::FileType type = file::GetFileType(dir_);
     if (type == file::FileType::kNonexistant) {
       diag->Error(DiagMessage() << "directory " << dir_ << " does not exist");