Link RELEASE_AVF_ENABLE_MULTI_TENANT_MICRODROID_VM to payload_not_root

Add plumbing in soong to conditionally enable payload_not_root rust cfg
if RELEASE_AVF_ENABLE_MULTI_TENANT_MICRODROID_VM build flag is set to true.

This patch also showcases how one can conditionally add files to
microdroid system image depending on the value of the build flag

Bug: 298008232
Test: build virt APEX with flag enabled
Test: verified payload runs as system_payload
Change-Id: I7e29b60a329ed4d7d8492bbe01b04ad4f895ff15
diff --git a/libs/microdroid_uids/Android.bp b/libs/microdroid_uids/Android.bp
index 497948d..ce62217 100644
--- a/libs/microdroid_uids/Android.bp
+++ b/libs/microdroid_uids/Android.bp
@@ -7,8 +7,7 @@
     crate_name: "microdroid_uids",
     srcs: ["src/lib.rs"],
     edition: "2021",
-    // TODO(b/296393106): Figure out how/when to enable this
-    // cfgs: ["payload_not_root"],
+    defaults: ["avf_build_flags_rust"],
     apex_available: [
         "com.android.virt",
     ],