blockimgdiff: selectively convert 'diff' commands to 'new' to reduce stash size

We cannot simultaneously stash more blocks than the size limit imposed by
the cache size. As a result, some 'diff' commands will be inevitably
converted to new. We used to do this conversion blindly when iterating
through the transfer list. This leads to an unintended large package.

In order to choose the right transfers to convert, we calculate the size
of the compressed data, and build a heuristic about the package size
increase to remove each stash blocks. After the process, the given
package size for the watch device further reduces from 186M->155M.

In some rare cases, the removed stashed blocks don't directly contribute
to the maximum simultaneously stashed size. For example,
stash A: 10 blocks
stash B: 5 blocks
free B: 5 blocks  <-- stash B has been freed before we reach max stashed blocks
stash C: 10 blocks

Converting these blocks lead to an uncertain result. On one hand, patches
are generally smaller than the new data; while on the other hand, the
regenerated graph may have fewer order violation and thus give some size
reduction. But these cases are rare and it seems an overkill to consider all
possible scenarios here.

Bug: 120561199
Test: build non-A/B incrementals and check the size
(p.s. it can be tested on all target files with customed cache threshold)
Change-Id: I599420a91b80f1a1d83d22ee1b336b699050cfb4