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/link/TableMerger.cpp b/tools/aapt2/link/TableMerger.cpp
index d808da3..7e7b9fb 100644
--- a/tools/aapt2/link/TableMerger.cpp
+++ b/tools/aapt2/link/TableMerger.cpp
@@ -24,6 +24,8 @@
 #include "ValueVisitor.h"
 #include "util/Util.h"
 
+using android::StringPiece;
+
 namespace aapt {
 
 TableMerger::TableMerger(IAaptContext* context, ResourceTable* out_table,
@@ -351,9 +353,8 @@
     const std::string& package, const FileReference& file_ref) {
   StringPiece prefix, entry, suffix;
   if (util::ExtractResFilePathParts(*file_ref.path, &prefix, &entry, &suffix)) {
-    std::string mangled_entry =
-        NameMangler::MangleEntry(package, entry.ToString());
-    std::string newPath = prefix.ToString() + mangled_entry + suffix.ToString();
+    std::string mangled_entry = NameMangler::MangleEntry(package, entry.to_string());
+    std::string newPath = prefix.to_string() + mangled_entry + suffix.to_string();
     std::unique_ptr<FileReference> new_file_ref =
         util::make_unique<FileReference>(
             master_table_->string_pool.MakeRef(newPath));