Microdroid: Add support for android15-6.6 GKI as guest kernel

Bug: 325991735
Test: build & install virt APEX with android15-6.6 GKI
Test: AVF presubmit tests
Change-Id: I16b5b4e0927cbddd000d17724319bcecd5ab6640
diff --git a/apex/sign_virt_apex.py b/apex/sign_virt_apex.py
index 55fe6d2..1a59894 100644
--- a/apex/sign_virt_apex.py
+++ b/apex/sign_virt_apex.py
@@ -498,7 +498,7 @@
     RunCommand(args, cmd)
 
 
-gki_versions = ['android14-6.1-pkvm_experimental']
+gki_versions = ['android14-6.1-pkvm_experimental', 'android15-6.6']
 
 # dict of (key, file) for re-sign/verification. keys are un-versioned for readability.
 virt_apex_non_gki_files = {