applypatch: Change the ssize_t length parameters to size_t.

Mostly for applypatch family APIs like ApplyBSDiffPatch() and
ApplyImagePatch(). Changing to size_t doesn't indicate they would
necessarily work with very large size_t (e.g. > ssize_t), just
similar to write(2). But otherwise accepting negative length doesn't
make much sense.

Also change the return type of SinkFn from ssize_t to size_t. Callers
tell a successful sink by comparing the number of written bytes against
the desired value. Negative return values like -1 are not needed. This
also makes it consistent with bsdiff::bspatch interface.

Test: recovery_component_test
Test: Apply an incremental with the new updater.
Change-Id: I7ff1615203a5c9854134f75d019e266f4ea6e714