releasetools: Specify a minimum size for right-sized images.

On aosp_blueline, system_other gets right-sized to 61,440 bytes. This
messes up the AVB footer calculation, and is too small to store an ext4
image.

Bug: 116608795
Test: build aosp_blueline w/ dynamic partitions
Change-Id: If0138a9d858765ff82cb957532504a066fd113ff