Update command processor and campaigns.

accept branch, build_target, pab_account_id argument from the leased job for gsi and vts.
added serial flag for flash command when shards == 1.
command_test.py will now can process --shards flag from 'run' context.

Test: > fetch <android-vts.zip>, test vts --shards 1 --serial <SERIAL>
Bug: 72474046
Change-Id: Ice3f1a4bf8dc2f93a0ca59e2d13bbf86bfad7341