Add virtual A/B compression product.

Sets ro.virtual_ab.compression.enabled and includes snapuserd_ramdisk.

A device will do the following to enable virtual a/b compression:

$(call inherit-product, $(SRC_TARGET_DIR)/product/virtual_ab_ota_compression.mk)

Bug: 168257347
Test: build with product included
Change-Id: Ide19576a69629389a7878d5dff3c5706c22527d9