Dedup snapshot module property set initialization

Previously, the code to add the compile_multilib property was
duplicated. This change moves the logic into the existing
addMemberPropertiesToPropertySet method, renames it to reflect new
behavior and renames and promotes the variantProperties struct from the
addSnapshotModule() method to the package level scope.

Bug: 186290299
Test: m nothing
Change-Id: I832a7fd89a3cf95467a1ff0807902c9be34c50b9