releasetools: Work around the issue with mke2fs created images.

When generating block based OTAs, we read files from the sparse image
directly with the help of block map file. However, the block map info
might not be accurate if the image is created with mke2fs. Because
mke2fs may skip allocating actual blocks if they contain all zeros.
ota_from_target_files.py consequently passes incomplete APK files to
imgdiff, which fails to generate patches.

This CL works around the issue by falling back from imgdiff to bsdiff on
failures. We should figure out a better way in b/68016761 to remove the
workaround, which would otherwise hide other issues in imgdiff.

Bug: 67824829
Bug: 68016761
Test: ota_from_target_files.py passes on previously failing TF zips.
Change-Id: Ib24c5b5f89812b97a0c87c6bf0dc147ae39bc92f