Allow sdk snapshot code to identify native bridge variants

Previously, the snapshot code ignored native bridge related arch
specific variants and only used the non native bridge variants. This
change enabled it to track native bridge variants too.

There is currently no support for creating native bridge specific
prebuilts so this ensures that any attempt to create one will fail
during generation of the snapshot with a clear message.

At the moment that error message cannot be tested by creating an sdk as
there is no way to add a dependency from the sdk onto a native bridge
variant. A follow up change will add the capability to test this
functionality through an sdk.

Bug: 195754365
Test: m nothing
Change-Id: I0494fdba9cc2ff013ea990d12d6a03a0a444bd5c