Export tombstones by default for debuggable VMs

VM config's export_tombstones is now optional. Default is true for
debuggable VMs, false for non-debuggable VMs.

Bug: 250165198
Test: atest MicrodroidHostTestCases
Change-Id: I818bcf3446b5bf7f560487265d82cd08f97c277e
diff --git a/virtualizationmanager/src/aidl.rs b/virtualizationmanager/src/aidl.rs
index cab7c71..678c91f 100644
--- a/virtualizationmanager/src/aidl.rs
+++ b/virtualizationmanager/src/aidl.rs
@@ -611,7 +611,7 @@
         apexes: vec![],
         extra_apks: vec![],
         prefer_staged: false,
-        export_tombstones: false,
+        export_tombstones: None,
         enable_authfs: false,
     })
 }