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/java/AnnotationProcessor.cpp b/tools/aapt2/java/AnnotationProcessor.cpp
index 2951e5c..a0ef00b 100644
--- a/tools/aapt2/java/AnnotationProcessor.cpp
+++ b/tools/aapt2/java/AnnotationProcessor.cpp
@@ -20,6 +20,8 @@
 
 #include "util/Util.h"
 
+using android::StringPiece;
+
 namespace aapt {
 
 void AnnotationProcessor::AppendCommentLine(std::string& comment) {
@@ -54,7 +56,7 @@
   for (StringPiece line : util::Tokenize(comment, '\n')) {
     line = util::TrimWhitespace(line);
     if (!line.empty()) {
-      std::string lineCopy = line.ToString();
+      std::string lineCopy = line.to_string();
       AppendCommentLine(lineCopy);
     }
   }