Unify OS APIs into one API

There are two APIs to specify a custom OS.
* 'os' field in a payload config json inside an APK
* 'osName' field in VirtualMachinePayloadConfig aidl parcelable

But that results in confusing and tricky APIs. For example
* VirtualMachineConfig.Builder.setOs() can be called only with
  VirtualMachineConfig.Builder.setPayloadBinaryName() because a payload
  config json already has its own OS field. It's confusing because users
  may misunderstand that customOS is only available to payload binaries.
* We need one payload config file per one supported VM, even when they
  are exactly same VMs except for the OS. This makes duplicated config
  files (especially for tests).

This change unifies those two OS APIs into one, under
VirtualMachineAppConfig.
* AIDL API users and VM APK developers: 'os' field in the json config
  will be deprecated and it will have no effect.
* Java API users: setOs must be called if they want to use OSes other
  than "microdroid".

Bug: 321130996
Test: atest MicrodroidHostTests MicrodroidTests AuthFsHostTest \
        CustomPvmfwHostTestCases DebugPolicyHostTests \
        ComposHostTestCases AVFHostTestCase PvmfwImgTest
Change-Id: I41a526c7001b4b9ff23a52bf612a996e114f292c
diff --git a/microdroid/payload/config/src/lib.rs b/microdroid/payload/config/src/lib.rs
index cdef3e4..d6f65bd 100644
--- a/microdroid/payload/config/src/lib.rs
+++ b/microdroid/payload/config/src/lib.rs
@@ -17,10 +17,12 @@
 use serde::{Deserialize, Serialize};
 
 /// VM payload config
-#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
+#[derive(Clone, Debug, Default, Deserialize, Eq, PartialEq, Serialize)]
 pub struct VmPayloadConfig {
-    /// OS config. Default: "microdroid"
+    /// OS config.
+    /// Deprecated: don't use. Error if not "" or "microdroid".
     #[serde(default)]
+    #[deprecated]
     pub os: OsConfig,
 
     /// Task to run in a VM
@@ -58,7 +60,7 @@
 
 impl Default for OsConfig {
     fn default() -> Self {
-        Self { name: "microdroid".to_owned() }
+        Self { name: "".to_owned() }
     }
 }