Use file extension with DfsPackDescription get/set file size.

Use file extension with DfsPackDescription get/set file size.

Previously the size getters and setters had explicit methods for index
and pack. Update the api to be based on the file extension. This will
make it possible to support other extensions in the future, such as
the forthcoming bitmap extensions.

Change-Id: Iab9d4abe0af65b2fc71ad71ef1db0feb6b3b5c58