Store/Pass root digests of APEX payload

Root digests of APEX payload are now collected by microdroid_manager and
stored in instance.img and passed to apexd.

Bug: 199371341
Test: MicrodroidHostTestCases
Change-Id: I940347068400822f7d8140c3daf68036f553b087
diff --git a/microdroid_manager/src/instance.rs b/microdroid_manager/src/instance.rs
index 73983a7..e50f4a8 100644
--- a/microdroid_manager/src/instance.rs
+++ b/microdroid_manager/src/instance.rs
@@ -328,5 +328,6 @@
 #[derive(Debug, Serialize, Deserialize, PartialEq)]
 pub struct ApexData {
     pub name: String,
-    pub pubkey: Vec<u8>,
+    pub public_key: Vec<u8>,
+    pub root_digest: Vec<u8>,
 }