AAPT2: Define and Implement AAPT Container Format

AAPT Container Format (.apc) is a simple container that
enumerates the various intermediate files that AAPT2 generates
during the compile phase.

The format is defined in formats.md.

For now, continue using the .flat extension for the container file,
and keep making use of the .flata zip for storing multiple files.
This will allow easier integration with existing build systems and allow
the evolution of the APC format to better handle arbitrarily large
files.

Test: make aapt2_tests
Change-Id: Id7216e5b76316bdd683f0fa4eaf2d2da273ba815
diff --git a/tools/aapt2/ResourceValues.h b/tools/aapt2/ResourceValues.h
index 742765d..fd242a1 100644
--- a/tools/aapt2/ResourceValues.h
+++ b/tools/aapt2/ResourceValues.h
@@ -249,6 +249,10 @@
   // This field is NOT persisted in any format. It is transient.
   io::IFile* file = nullptr;
 
+  // FileType of the file pointed to by `file`. This is used to know how to inflate the file,
+  // or if to inflate at all (just copy).
+  ResourceFile::Type type = ResourceFile::Type::kUnknown;
+
   FileReference() = default;
   explicit FileReference(const StringPool::Ref& path);