releasetools: Add Payload class.

This breaks down the current WriteABOTAPackageWithBrilloScript() into
smaller and testable units, which also prepares for the work in
b/35724498.

Bug: 35724498
Test: python -m unittest test_ota_from_target_files
Test: Get identical A/B OTA packages w/ and w/o the CL.
Change-Id: I2ea45ce98e2d2baa58e94fb829b7242f6fe685a7