Temporarily use blockimgdiff v2 for OTA.

angler and bullhead have smaller cache partiton (~100MB) and hit issue
with incremental OTA (such as MDA09B to MDA12) that requires more stash
space with BBOTA v3. Temporarily use v2 as a workaround, until we come
up with a real fix. Note that BBOTA v2 still uses some stash space, but
should be much smaller than v3.

Bug: 22430577
Change-Id: Ic4c05f3609732e7d5140703062da9fe11df5c6d3