Fix VendorTagId to be metadata_vendor_id_t

- It is currently int. This was causing downstream issues
  because the CameraMetadata object passed to
  configureStreams had the wrong vendor_id, so we couldn't
  call update() on it with vendor tags.
- As a drive by, specify getVendorId() as being const.
  Without this, we need to keep copying const CameraMetadata
  objects to simply get their vendor id.

Thanks epeev@ for all your pointers along the debugging
journey!

Bug: 316413287
Test: m
Change-Id: I32bf9bcb7137990d28b9fcbe1b27bc41ebbd50ee