OTA: Support A/B devices custom images update.

Add a new custom_image option to configure which custom images to use to
update custom partitions in A/B update.

This change also moves oem_settings to common option as A/B update will
use it to set oem properties too.

BUG: 171225290
Test: unittest pass, generate OTAs, flash to devices and check results
Change-Id: I279477d6b2954fb3705d7efede0a8bcd330c108b