Retry: Fix snapshot of a host/device cc_library with stubs

Change since last attempt: Disable test that breaks on darwin.

Adds a test that fails with unknown property android.stubs.versions
and then fixes that by marking the field from which that property is
created with 'ignored-on-host' and implemented the isHostVariant on
*osTypeSpecificInfo.

Bug: 155628860
Test: m nothing
Change-Id: Ia4e744c9e799d5adaf8a2f761516f568ec363ad4
diff --git a/sdk/update.go b/sdk/update.go
index 991428e..d43a42d 100644
--- a/sdk/update.go
+++ b/sdk/update.go
@@ -982,6 +982,13 @@
 	}
 }
 
+func (osInfo *osTypeSpecificInfo) isHostVariant() bool {
+	osClass := osInfo.osType.Class
+	return osClass == android.Host || osClass == android.HostCross
+}
+
+var _ isHostVariant = (*osTypeSpecificInfo)(nil)
+
 func (osInfo *osTypeSpecificInfo) String() string {
 	return fmt.Sprintf("OsType{%s}", osInfo.osType)
 }