Fix ExcludeFromVendorSnapshot check

ExcludeFromVendorSnapshot() must return true if the module must not
be snapshotted and built from the vendor source tree.
The default value must be false because rust modules are provided by
the system source tree.

Bug: 184042776
Bug: 189372652
Test: m nothing
Change-Id: I267c3a5674bea966d1dd831cda73538ae94cf25f