Add virtual A/B product.

- Add PRODUCT_VIRTUAL_AB_OTA(_RETROFIT)
- Set ro.virtual_ab.{enabled,retrofit} respectively.

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

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

Bug: 135752105
Test: build with device with flag set
Change-Id: I6cb1207aef3ddcc40e215e20589abf995ec81aac