DO NOT MERGE: Add the option to reserve headroom for

partition images. This is useful for devices 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.

Change-Id: I048b45e4b7b68ed2e855ea852b09ee1fdc6ad35d