Build system_other image in soong-only builds

This just builds an empty system_other image for now, the dexpreopt
files will be added in a followup change.

Bug: 390269431
Test: m --soong-only
Change-Id: Ic4a9bcb8b7ba1eb4444b3339d6c0b0cdfd485714
diff --git a/android/variable.go b/android/variable.go
index 08bcedf..14094e2 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -623,6 +623,8 @@
 	VendorDlkmSecurityPatch string `json:",omitempty"`
 	OdmDlkmSecurityPatch    string `json:",omitempty"`
 
+	BuildingSystemOtherImage bool `json:",omitempty"`
+
 	// Boot image stuff
 	BuildingRamdiskImage              bool     `json:",omitempty"`
 	ProductBuildBootImage             bool     `json:",omitempty"`