AAPT2: Separate out the various steps

An early refactor. Some ideas became clearer as
development continued. Now the various phases are much
clearer and more easily reusable.

Also added a ton of tests!

Change-Id: Ic8f0a70c8222370352e63533b329c40457c0903e

tools/aapt2/ConfigDescription.cpp

@@ -17,8 +17,8 @@
 #include "ConfigDescription.h"
 #include "Locale.h"
 #include "SdkConstants.h"
-#include "StringPiece.h"
-#include "Util.h"
+#include "util/StringPiece.h"
+#include "util/Util.h"
 
 #include <androidfw/ResourceTypes.h>
 #include <string>