[rkp] Return error code when requestAttestation fails

This cl converts the return value of
IVmPayloadService#requestAttestation from a binder status to
an error code defined in attestation_status_t.
In the event of an error, this value is now sent back to the
client VM payload.

Previously, the API would panic when encountering an error.

Bug: 309790130
Test: Run ServiceVmClientTestApp
Change-Id: I585e3553e2783ca019d2b8076671404398bde447
diff --git a/vm_payload/include/vm_payload.h b/vm_payload/include/vm_payload.h
index 2dfa2cb..951b57f 100644
--- a/vm_payload/include/vm_payload.h
+++ b/vm_payload/include/vm_payload.h
@@ -43,11 +43,14 @@
     /** The remote attestation completes successfully. */
     ATTESTATION_OK = 0,
 
-    /** The remote attestation has failed due to an unspecified cause. */
-    ATTESTATION_UNKNOWN_ERROR = -10000,
-
     /** The challenge size is not between 0 and 64. */
     ATTESTATION_ERROR_INVALID_CHALLENGE = -10001,
+
+    /** Failed to attest the VM. Please retry at a later time. */
+    ATTESTATION_ERROR_ATTESTATION_FAILED = -10002,
+
+    /** Remote attestation is not supported in the current environment. */
+    ATTESTATION_ERROR_UNSUPPORTED = -10003,
 } attestation_status_t;
 
 /**