releasetools: Clean up the use of `partition_size`.

Unless using dynamic partitions, `partition_size` should be a fixed
value that equals to the partition size in BoardConfig. It should stay
the same when building an image for that partition. Only the actual
image size that's used to hold the filesystem could be adjusted. This CL
cleans up the uses of `partition_size` and `image_size` to better
reflect such logic.

With dynamic partitions, the only thing that changes is the need to
compute `partition_size` upfront. Once that's done, `partition_size`
should remain unchanged.

Test: `m dist`
Test: `python -m unittest test_add_img_to_target_files`
Test: `python -m unittest test_validate_target_files`
Change-Id: Idedb3e018c95e8f63dc4d9c423be27f30ebb584f