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/StringPool.cpp b/tools/aapt2/StringPool.cpp
index 3032829..d968d73 100644
--- a/tools/aapt2/StringPool.cpp
+++ b/tools/aapt2/StringPool.cpp
@@ -22,11 +22,13 @@
 
 #include "android-base/logging.h"
 #include "androidfw/ResourceTypes.h"
+#include "androidfw/StringPiece.h"
 
 #include "util/BigBuffer.h"
-#include "util/StringPiece.h"
 #include "util/Util.h"
 
+using android::StringPiece;
+
 namespace aapt {
 
 StringPool::Ref::Ref() : entry_(nullptr) {}
@@ -140,7 +142,7 @@
   }
 
   Entry* entry = new Entry();
-  entry->value = str.ToString();
+  entry->value = str.to_string();
   entry->context = context;
   entry->index = strings_.size();
   entry->ref_ = 0;