Dedupe on-demand splits before adding to primary splits

Some android modules use `split_all_variants: true` to add the on-demand
variants to the primary variants. Deduping this in
blueprint/transition.go ensures that we do not create duplicate splits.

Test: m nothing --no-skip-soong-tests
Test: SOONG_SPLIT_ALL_VARIANTS=true m nothing
Change-Id: I5d1ab3508f49b2b1278768cbd057b3ef1e3d1b74