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/io/FileSystem.cpp b/tools/aapt2/io/FileSystem.cpp
index 828f34e..027cbd0 100644
--- a/tools/aapt2/io/FileSystem.cpp
+++ b/tools/aapt2/io/FileSystem.cpp
@@ -16,14 +16,16 @@
 
 #include "io/FileSystem.h"
 
+#include "androidfw/StringPiece.h"
 #include "utils/FileMap.h"
 
 #include "Source.h"
 #include "util/Files.h"
 #include "util/Maybe.h"
-#include "util/StringPiece.h"
 #include "util/Util.h"
 
+using android::StringPiece;
+
 namespace aapt {
 namespace io {
 
@@ -54,13 +56,11 @@
 }
 
 IFile* FileCollection::InsertFile(const StringPiece& path) {
-  return (files_[path.ToString()] =
-              util::make_unique<RegularFile>(Source(path)))
-      .get();
+  return (files_[path.to_string()] = util::make_unique<RegularFile>(Source(path))).get();
 }
 
 IFile* FileCollection::FindFile(const StringPiece& path) {
-  auto iter = files_.find(path.ToString());
+  auto iter = files_.find(path.to_string());
   if (iter != files_.end()) {
     return iter->second.get();
   }