Move extent copy and hash logic to a new file.

The SOURCE_COPY operation used to copy the source blocks one by one to
the target partition. This process is sub-optimal if there are several
consecutive blocks. This patch moves this copy and hash logic to a new
file and adds several unittests for it. The new logic copies in chunks
of up to 1MiB when the source and target data is contiguous.

Bug: 34284069
Test: Added unittests.

Change-Id: I9ed52b429a54a2b4d6edaba051284b7dcd8a9525