Remove old key management

Delete a load of no longer needed code.

We no longer support persisting keys in the host. CompOS no longer
accesses DICE directly (compos_key_helper handles that).

We retain the instance image files, but rename pending to current
(it's created before reboot with the staged APEXes and used after
reboot with the current APEXes, but there's no point renaming it).

Remove the attempt to start an existing instance when running
compilation - it is slow, and vanishingly unlikely to work.

Sadly this also deletes all the CompOS unit tests. (But there are some
new ones in compos_key_tests.)

Bug: 218494522
Test: Manual; atest ComposTestCase; atest CompOsSigningHostTest
Change-Id: I0175270341d5dcad614106432b7d2650229cf8a6
diff --git a/compos/composd/src/service.rs b/compos/composd/src/service.rs
index 8e5586e..f4798d7 100644
--- a/compos/composd/src/service.rs
+++ b/compos/composd/src/service.rs
@@ -70,8 +70,7 @@
         &self,
         callback: &Strong<dyn ICompilationTaskCallback>,
     ) -> Result<Strong<dyn ICompilationTask>> {
-        // TODO: Try to start the current instance with staged APEXes to see if it works?
-        let comp_os = self.instance_manager.start_pending_instance().context("Starting CompOS")?;
+        let comp_os = self.instance_manager.start_current_instance().context("Starting CompOS")?;
 
         let target_dir_name = PENDING_ARTIFACTS_SUBDIR.to_owned();
         let task = OdrefreshTask::start(