virtualizationservice: "prefer_staged" for apexes

App can tell virtualizationservice to use "staged" apexes for its
payload.

  // vm_config.json
  {
    ..
    "perfer_staged": true
  }

Bug: 199146189
Test: MicrodroidHostTestCases
Test: adb install --staged new-adbd.apex
      vm run-app with prefer_staged: true, started with staged adbd apex
Change-Id: I6320247362e07519e120aed19b290618270e1335
diff --git a/microdroid/payload/config/src/lib.rs b/microdroid/payload/config/src/lib.rs
index 6dc127b..2547f3d 100644
--- a/microdroid/payload/config/src/lib.rs
+++ b/microdroid/payload/config/src/lib.rs
@@ -30,6 +30,10 @@
     /// APEXes to activate in a VM
     #[serde(default)]
     pub apexes: Vec<ApexConfig>,
+
+    /// Tells VirtualizationService to use staged APEXes if possible
+    #[serde(default)]
+    pub prefer_staged: bool,
 }
 
 /// OS config