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/microdroid/payload/config/src/lib.rs b/microdroid/payload/config/src/lib.rs
index 08b8b42..925a543 100644
--- a/microdroid/payload/config/src/lib.rs
+++ b/microdroid/payload/config/src/lib.rs
@@ -40,8 +40,8 @@
     pub prefer_staged: bool,
 
     /// Whether to export the tomsbtones (VM crashes) out of VM to host
-    /// This does not have a default & the value is expected to be in json for deserialization
-    pub export_tombstones: bool,
+    /// Default: true for debuggable VMs, false for non-debuggable VMs
+    pub export_tombstones: Option<bool>,
 
     /// Whether the authfs service should be started in the VM. This enables read or write of host
     /// files with integrity checking, but not confidentiality.