Add the option to reserve headroom for partition

images. This is useful for device with low disk
space with different build variants.

This is a manual cherry-pick from aosp/master.

Bug: 37469715
Test: regular image builds successfully, errors occur
when the headroom size is greater than available
partition space.

Merged-In: I526cdd0f84981bbd16e3afcfe1cd7fc43dce98ef

Change-Id: I840d212bb46612074c43b39e94546901f60e4b4c