Reduce PatchWriterInterface functionality.

The recently introduced PatchWriterInterface had both the patch file
format logic and the diff/extra streams selection from the control
entries. While this simplifies the bsdiff main algorithm and makes
invalid usages of the BsdiffPatchWriter evident, writing alternative
PatchWriterInterface classes required to replicate the diff/extra
stream selection logic.

This patch splits out the diff/extra stream generation and all the
checks around those to a new helper class DiffEncoder. The public
interface PatchWriterInterface now has two methods to accept the diff
and extra stream data, and does not compute them.

Bug: 34220646
Test: Added unittests. Ran bsdiff on some pairs of files obtaining the same result as before.
Change-Id: I5f303c06f1e10910eb00dcfda38c6811977a91cf