Move ART cpu properties to /vendor

System properties relating to the physical hardware belong
on the vendor partition. Move the ART-related cpu variant
sysprops there.

This also brings the ART sysprops in line with the equivalent
bionic sysprops.

Bug: 120773446
Test: make, boot
Test: grep dalvik.vm vendor/default.prop
Change-Id: I3f843a7070dd0d9148737e040fe87d6461977d09