Remove the assumption of target chunk size in imgdiff

In the split mode of imgdiff, we used to assume that the size of a split
target chunk is always greater than the blocksize i.e. 4096. This may
lead to the following assertion failure:
I0221 04:57:33.451323 818464 common.py:205 imgdiff F 02-21 04:57:33 821203 821203 imgdiff.cpp:999]
Check failed: tgt_size >= BLOCK_SIZE (tgt_size=476, BLOCK_SIZE=4096)

This CL removes the assumption and handles the edge cases.

Test: generate and verify the incremental update for TFs in the bug; unit test passes

Bug: 73757557
Bug: 73711365
Change-Id: Iadbb4ee658995f5856cd488f3793980881a59620