releasetools: Disable using imgdiff for squashfs.

We use imgdiff to handle files in zip format (e.g. jar/zip/apk) for
higher compression ratio.

For system/vendor in squashfs, a) all files are compressed in LZ4
format; b) we use 4096-byte block size in their sparse images, but the
files in squashfs may not be laid out as 4K-aligned. So the blocks for
a given file as listed in block map may not form a valid zip file, which
may fail the patch generation with imgdiff.

Disable using imgdiff for squashfs images, and use bsdiff instead.

Bug: 22322817
Change-Id: Ie76aa4cece5c9d38cb1d1a34c505a4a8f37512d3