Adds validation check that certain partitions come from a single build.

This is to prevent a user from accidentally including files from the
wrong build. For example, adding any SYSTEM/ line to other_item_list
while keeping SYSTEM/* in system_item_list would cause the other build
to introduce an extra or changed file in the system image.

Bug: 132730710
Test: python -m unittest test_merge_target_files
Change-Id: Ic1178cdc9b991114f293ff3f2b4e6054e06647c6