Initial work to make VM config file optional

Modify VirtualMachineAppConfig to allow configPath to be omitted;
instead a subset of the available config options can be specified
directly.

Modify VS to map the config options to the Metadata proto file.

At this stage nothing ever sets the payload config directly, and
Microdroid would reject it if it received it, but this at least
establishes the plumbing.

Bug: 243513572
Test: atest MicrodroidTests
Change-Id: I596384abf6e3bbbccd8934ba28fa3a85faa51b56
diff --git a/vm/src/main.rs b/vm/src/main.rs
index 0845897..21cc74b 100644
--- a/vm/src/main.rs
+++ b/vm/src/main.rs
@@ -48,7 +48,7 @@
         instance: PathBuf,
 
         /// Path to VM config JSON within APK (e.g. assets/vm_config.json)
-        config_path: String,
+        config_path: Option<String>,
 
         /// Name of VM
         #[clap(long)]
@@ -205,7 +205,7 @@
             &apk,
             &idsig,
             &instance,
-            &config_path,
+            config_path.as_deref().unwrap_or(""),
             daemonize,
             console.as_deref(),
             log.as_deref(),