Generate snapshot member list properties from dependencies

Previously, the snapshot member list properties were simply copied from
the sdk variants. That worked because the snapshot member list
properties must include all exported (i.e. visible outside the sdk
snaphot) members and only members explicitly specified in the sdk
member list properties were exported. However, a preceding change
allowed members that were transitively added (and so not explicitly
specified on the sdk) to be exported.

This change ensures that those exported members are added to the
snapshot member properties which means that they will now work properly
with the sdk version mutators.

This does not add tests for this because no existing SdkMemberType
exports its transitive dependencies and there is currently no way to
register an SdkMemberType for testing only. A following change will
exercise this functionality.

Bug: 186290299
Test: m nothing
Change-Id: Iadbdd86f6dcdd183fa99f647222a43d412271501