Add support merging system and other for the non-A/B configuration.

For non-A/B, you must supply the --system-item-list, --other-item-list, and
--system-misc-info-keys parameters approrpriate for merging two partial target
files. Additionally, you must supply the --rebuild-recovery option to correctly
generate the filesystem config and file contexts. With all of these parameters,
the script will generate a merged target files containing the correct recovery
files.

Also fix the --keep-tmp option to be consistent with the other options.

Bug: 122813742
Test: verify that merged target image boots and can perform OTA.
Change-Id: I5a942ac0cd9924fec419a686794a2340304594c8