Most patch formats include the size of the new file in the header.
To help streaming the patch to disk while generating it, this CL
passes the size of the new file to the patch writer on initialization.

To do this, we also move the Init() call to the patch writer to the
DiffEncoder, which makes more sense since the Close() call is also made
from the DiffEnconder.

Bug: None
Test: Updated tests to check for this value.
Change-Id: Idfaedbd492d68ab6e6cb2c1cb3883947f068c3aa