Flag guard the code related to the vendor modules feature

The feature is guarded by the RELEASE_AVF_ENABLE_VENDOR_MODULES flag. On
the virtmgr side the flag-guard check is done in the create_vm_internal
function. On the vm binary side the related flags (--vendor, --kernel)
are only enabled if the RELEASE_AVF_ENABLE_VENDOR_MODULES flag is
enabled.

Additionally the vendor modules related tests in MicrodroidTests now run
conditionally depending on the result of the
`isFeatureEnabled(FEATURE_VENDOR_MODULES)` call.

Bug: 298007909
Test: atest MicrodroidTests
Change-Id: I1467194c802720601a10d0a760a8f1d7ce134037
diff --git a/Android.bp b/Android.bp
index 1fae793..d1086ba 100644
--- a/Android.bp
+++ b/Android.bp
@@ -34,6 +34,7 @@
     config_namespace: "ANDROID",
     bool_variables: [
         "release_avf_enable_multi_tenant_microdroid_vm",
+        "release_avf_enable_vendor_modules",
     ],
     properties: [
         "cfgs",
@@ -46,5 +47,8 @@
         release_avf_enable_multi_tenant_microdroid_vm: {
             cfgs: ["payload_not_root"],
         },
+        release_avf_enable_vendor_modules: {
+            cfgs: ["vendor_modules"],
+        },
     },
 }