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_test.cpp b/tools/aapt2/StringPool_test.cpp
index e1394fc..f64a8cf 100644
--- a/tools/aapt2/StringPool_test.cpp
+++ b/tools/aapt2/StringPool_test.cpp
@@ -18,9 +18,14 @@
 
 #include <string>
 
+#include "androidfw/StringPiece.h"
+
 #include "test/Test.h"
 #include "util/Util.h"
 
+using android::StringPiece;
+using android::StringPiece16;
+
 namespace aapt {
 
 TEST(StringPoolTest, InsertOneString) {
@@ -236,8 +241,7 @@
     EXPECT_EQ(StringPiece16(u"goodbye"), util::GetString16(test, 1));
 
     EXPECT_EQ(StringPiece(sLongString), util::GetString(test, 2));
-    EXPECT_EQ(util::Utf8ToUtf16(sLongString),
-              util::GetString16(test, 2).ToString());
+    EXPECT_EQ(util::Utf8ToUtf16(sLongString), util::GetString16(test, 2).to_string());
 
     size_t len;
     EXPECT_TRUE(test.stringAt(3, &len) != nullptr ||