blockimgdiff: add a function to select and convert 'diff' to new

Package size will be unintended large if we stash more blocks than the
stash limit specified by the cache size. To reduce the maximum size of
simultaneous stashed blocks, we will inevitably convert some 'diff'
commands to 'new' commands.

To mitigate the impact, we add a new function to smartly select the
transfers to convert based on their patch size and compressed size.
This cl converts the transfers that have a larger patch size than the
compressed target sizes. And there's a slightly improvement in the
final package size: from 194M -> 185M.

Bug: 120561199
Test: build a non-A/B incremental package, run simulator
Change-Id: Id73ff736ba4e6901d245ad5549d42310d0740284