releasetools: Really allow removing partitions

- Add hashtree_info to EmptyImage so that BlockDifference.Compute()
can accept EmptyImage() as target image, which is the case when
a partition is removed.

- BlockDifference also checks source_info_dict to determine
whether a partition is dynamic. When a partition is removed,
its name does not appear in target_info_dict.

- Add tests to ensure DynamicPartitionDifference() still works.

Test: DynamicPartitionDifferenceTest
Test: test_blockimgdiff

Change-Id: Iadb1db075f5dc344db6d5ade358c83b01231e443