sm8250-common: Migrate to restructured sepolicy_vndr

The new folder structure now looks like this:

  device/qcom/sepolicy_vndr
  ├─ legacy-um (meant for sdm845 until sm8350)
  ├─ sm8450
  └─ sm8550

SEPolicy.mk placed in the root automatically redirects devices to the
proper subfolder.

Change-Id: I4ed1a06af779909d64172da294c4a99a3ae9ba0d