Allow merging target files without framework ab_partitions.txt

Introduce a new option `--allow-partial-ab` in merge_target_files,
which allows merging a non-AB framework with an AB vendor.

The reason for adding this option is to support merging a real
device framework with a cuttlefish vendor. Cuttlefish enables AB
partition by default; however, some real devices do not.

Bug: 318326532
Test: merge_target_files
Test: atest --host releasetools_test
Change-Id: Iaebd06796153fe82fbf56e86fcc8c500b6d60771