Always include host variants in the sdk snapshot

This relands aosp/2478277. The original CL was submitted as part of a
stack that broke tm and udc since those branches contains soong modules
that have min_sdk_version of type (kind+level).

Test: go test ./sdk
Bug: 208456999
Change-Id: Ie5d7f2597092a51c6e74e7b3cab2f81c02906d5e