Remove direct access to the sealing CDI from the payload

Change the API from offering the raw sealing CDI to offering VM instance
secrets that happend to be derived from the sealing CDI. This makes it
harder for the payload to leak its sealing CDI and losing the ability to
have secrets in the VM.

Bug: 243514248
Test: atest MicrodroidTests
Test: atest ComposHostTestCases
Change-Id: I0e72dabe7daca4d72a35788412d2ee19a3b446a5
diff --git a/microdroid/vm_payload/src/lib.rs b/microdroid/vm_payload/src/lib.rs
index e3da227..74dd8f4 100644
--- a/microdroid/vm_payload/src/lib.rs
+++ b/microdroid/vm_payload/src/lib.rs
@@ -17,6 +17,6 @@
 mod vm_service;
 
 pub use vm_service::{
-    get_dice_attestation_cdi, get_dice_attestation_chain, get_dice_sealing_cdi,
+    get_dice_attestation_cdi, get_dice_attestation_chain, get_vm_instance_secret,
     notify_payload_ready,
 };