Fix .vendor suffix for snapshots

.vendor suffix is meant to be attached for vendor variants of
modules without "vendor: true". So when we decide whether the suffix is
needed for a snapshot module, we should check if corresponding source
module is "vendor: true" or not, instead of checking vendor_available of
the source module.

Bug: 149252327
Test: manual
Change-Id: Ibde75737ddce8e1dc974453e88832f9272a2c66e