Add os parameter to composd_cmd

This will be used by AVFHostTestCases and ComposTestCases to test
various microdroid OSes.

Bug: 302465542
Test: composd_cmd
Change-Id: I85b86fa66f0c79b633eb359873b94edcff79d561
diff --git a/android/composd/src/instance_manager.rs b/android/composd/src/instance_manager.rs
index 9e94035..cb1f7e4 100644
--- a/android/composd/src/instance_manager.rs
+++ b/android/composd/src/instance_manager.rs
@@ -46,11 +46,12 @@
         self.start_instance(CURRENT_INSTANCE_DIR, vm_parameters)
     }
 
-    pub fn start_test_instance(&self, prefer_staged: bool) -> Result<CompOsInstance> {
+    pub fn start_test_instance(&self, prefer_staged: bool, os: &str) -> Result<CompOsInstance> {
         let mut vm_parameters = new_vm_parameters()?;
         vm_parameters.name = String::from("ComposdTest");
         vm_parameters.debug_mode = true;
         vm_parameters.prefer_staged = prefer_staged;
+        vm_parameters.os = os.to_owned();
         self.start_instance(TEST_INSTANCE_DIR, vm_parameters)
     }