Add multithread support to call imgdiff with block-limit

With the new implementation of handling large apks, we need to call
imgdiff with block-limit to split the apk and generate the patch at
the same time. The call to imgdiff would significantly increase the
time consumption of the "FindTransfers" function which we used to
execute sequentially. This cl addresses this issue and speeds up the
process by making the imgdiff call parallel.

Bug: 34220646
Test: Create and sideload an incremental package for angler
Change-Id: Id62e348418fc1d22e32ea6c8ac16d9ab3ec92d7b