Rename ro.vendor.product.* to ro.product.vendor.*

"ro.vendor." prefix will be used only for vendor-specific properties
except for ro.vendor.build.date, ro.vendor.build.date.utc and
ro.vendor.build.fingerprint.

Those exceptions have been used since Android L. So we can't rename them
to support the system-only upgrade from old Android.

Bug: 36796459
Test: confirmed that ro.vendor.product.* were changed to
ro.product.vendor.* in vendor/build.prop

Change-Id: Ibc107b00264a2dc194261d1968b27a9b17b5d798