releasetools: Reduce memory footprint for BBOTA generation.

The major issue with the existing implementation is unnecessarily
holding too much data in memory, such as HashBlocks() which first reads
in *all* the data to a list before hashing. We can leverage generator
functions to stream such operations.

This CL makes the following changes to reduce the peak memory use.
 - Adding RangeSha1() and WriteRangeDataToFd() to Image classes. These
   functions perform the operations on-the-fly.
 - Caching the computed SHA-1 values for a Transfer instance.

As a result, this CL reduces the peak memory use by ~80% (e.g. reducing
from 5.85GB to 1.16GB for the same incremental, as shown by "Maximum
resident set size" from `/usr/bin/time -v`). It also effectively
improves the (package generation) performance by ~30%.

Bug: 35768998
Bug: 32312123
Test: Generating the same incremental w/ and w/o the CL give identical
      output packages.
Change-Id: Ia5c6314b41da73dd6fe1dbe2ca81bbd89b517cec