Really support removing / adding partitions.

- When removing a partition, BlockDifference() object
will have tgt=EmptyImage(). Fix the asserts accordingly.
Also, BlockDifference object now allow tgt=None case.

- When adding a partition, BlockDifference() object
will have src=None. Fix the asserts accordingly.

Also, add unit tests to DynamicPartitionsDifference.

Test: create incremental OTA
Test: test_common.DynamicPartitionsDifferenceTest
Bug: 111801737

Change-Id: I3a35378ecf93111b8f44545cff6ae9696b6b4851