releasetools: Refactor the condition checking for using imgdiff.

In Transfer class, unbundle 'intact' with the monotonicity of the input
ranges. Negate the logic of 'intact', and thus rename it to 'trimmed'.
Move this property from an attribute of Transfer class as the one in
RangeSet.extra. 'trimmed' indicates whether the source / target ranges
have been modified after creating the Transfer() instance.

The logic that determines whether we can apply imgdiff has been
refactored and consolidated into BlockImageDiff.CanUseImgdiff(). Now
both of the two paths call this single copy, i.e. the one that detects
large APKs (before creating Transfer()'s), and the other that's about to
generate the patch for a given Transfer instance.

Bug: 68016761
Test: python -m unittest test_blockimgdiff
Test: Generate an incremental BBOTA package.
Change-Id: Id07195f63f1fa6c3af6e9091940d251cf09fa104