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/Resource.cpp b/tools/aapt2/Resource.cpp
index 3eef7aa7..fdabce1 100644
--- a/tools/aapt2/Resource.cpp
+++ b/tools/aapt2/Resource.cpp
@@ -19,6 +19,8 @@
 #include <map>
 #include <string>
 
+using android::StringPiece;
+
 namespace aapt {
 
 StringPiece ToString(ResourceType type) {