delta_generator: Use REPLACE_XZ in deltas when allowed.

This patch moves the generation of the best allowed full operation to a
new function which now includes ZERO, REPLACE, REPLACE_BZ and REPLACE_XZ
when allowed. This function is used to produce a full operation
whenever it is needed in the "full" payload generator and the AB "delta"
payload generator.

Bug: 24578399
TEST=brillo_update_payload generated REPLACE_XZ operations for a delta payload that was using REPLACE_BZ operations and merged them.
TEST=brillo_update_payload generated a full payload with REPLACE, REPLACE_BZ and REPLACE_XZ operations.
TEST=Updated unittests.

Change-Id: I29618094a84ef8050f8dd7ede0fd70056dc9d650