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

sdk/update.go

@@ -357,6 +357,12 @@ func (s *sdk) buildSnapshot(ctx android.ModuleContext, sdkVariants []*sdk) {
 		// If the minApiLevel of the member is greater than the target API level then exclude it from
 		// this snapshot.
 		exclude := memberVariantDep.minApiLevel.GreaterThan(targetApiLevel)
+		// Always include host variants (e.g. host tools) in the snapshot.
+		// Host variants should not be guarded by a min_sdk_version check. In fact, host variants
+		// do not have a `min_sdk_version`.
+		if memberVariantDep.Host() {
+			exclude = false
+		}
 
 		addMember(name, export, exclude)
 
@@ -1263,6 +1269,11 @@ type sdkMemberVariantDep struct {
 	minApiLevel android.ApiLevel
 }
 
+// Host returns true if the sdk member is a host variant (e.g. host tool)
+func (s *sdkMemberVariantDep) Host() bool {
+	return s.variant.Target().Os.Class == android.Host
+}
+
 var _ android.SdkMember = (*sdkMember)(nil)
 
 // sdkMember groups all the variants of a specific member module together along with the name of the