Always include host variants in the sdk snapshot

Host tools (and variants in general) should not fall under the purview
of min_sdk_version check. These do not exist on device, so
min_sdk_version check would not make such sense.

This is prework for migrating min_sdk_version from type SdkSpec to
ApiLevel.

Test: go test ./sdk
Bug: 208456999

Change-Id: I2fd822a223d1dc5e290d4a1ebf304fe47a5b0416