Move device specific properties to /vendor

By setting BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED on aosp_arm64_ab
the device specific properties like ro.product.board
are hosted in /vendor/build.prop.

Test: build and flash aosp_arm64_ab system.img on sailfish and marlin.
Change-Id: I8062ff81221a3026626736b012ceaf99d8ca2d12