Split large files for BBOTA v3.

For BBOTA v3, we need to stash source blocks to support resumable
feature. However, with the growth of file size and the shrink of the
cache size, source blocks that represent a file are too large to be
stashed as a whole. CL in [1] solves the issue by replacing the diff
command with a "new" command. However, it may increase the generated
package size substantially (e.g. from ~100MB to ~400MB).

With this CL, if a file spans too many blocks, we split it into smaller
pieces by generating multiple commands. For the same case above, it
reduces the package size to ~150MB.

One potential downside is that after splitting, files like .jar,
.apk and .zip can no longer use imgdiff. We may lose the potential
benefit of using imgdiff for patch size reduction.

[1] commit 82c47981

Bug: 22430577
Change-Id: I5684fab0ac41c8d94fe18f52a34cef77e06f741c