Rewrite key management & signing

Extend compos_helper to support signing, use it from CompOS.

Expose the public key from the VM. Rename compos_verify_key to
compos_verify and get it to verify the signature against the current
instance's public key.

Also move DICE access to compos_key_main. There's no use having it in
the library - neither the tests nor compos_verify can use it - and it
complicates the build rules.

There's a lot more that can be deleted, but I'll do that in a
follow-up; this is big enough already.

Bug: 218494522
Test: atest CompOsSigningHostTest CompOsDenialHostTest
Change-Id: I2d71f68a595d5ddadb2e7b16937fa6855f5db0ab
diff --git a/compos/common/timeouts.rs b/compos/common/timeouts.rs
index e6cc430..b3ec1e5 100644
--- a/compos/common/timeouts.rs
+++ b/compos/common/timeouts.rs
@@ -56,7 +56,7 @@
     // Note: the source of truth for these odrefresh timeouts is art/odrefresh/odr_config.h.
     odrefresh_max_execution_time: Duration::from_secs(300),
     odrefresh_max_child_process_time: Duration::from_secs(90),
-    vm_max_time_to_ready: Duration::from_secs(15),
+    vm_max_time_to_ready: Duration::from_secs(20),
 };
 
 /// The timeouts that we use when need_extra_time() returns true.