libsnapshot: Off-line tool for converting OTA payloads to COWs.

The A/B version of this tool takes in a payload.bin file (from an OTA
package), and converts it into COWs in the new format. It uses the
CowWriter API to do this. This tool can be used to precisely see the
size of a COW relative to an OTA. Since there is one COW per partition,
it also takes an output folder:

    make_cow_from_ab_ota payload.bin ./ota-cows/

If the payload is incremental, a source target-files package is needed.
It must be the exact package used to build the OTA, otherwise, the
conversion is likely to fail. Example:

    make_cow_from_ab_ota -source_tf target-files-XYZ.zip \
                         payload.bin ./ota-cows/

Bug: 162274240
Test: manual tests
Change-Id: I8cb1554e71384625c8073f1c351a976b5ae00a36