Define and implement a service for key management in CompOS.

This is based on the interface prototyped in
system/security/ondevice-signing/FakeCompOs.h.

For now, to allow manual testing, this is a standalone binary.

Bug: 191763370
Test: Builds.
Change-Id: I307ba9144fa51cca7ebee2142980f3a1cd436ef2
diff --git a/compos/apex/Android.bp b/compos/apex/Android.bp
index 3a8f601..95463d0 100644
--- a/compos/apex/Android.bp
+++ b/compos/apex/Android.bp
@@ -36,6 +36,7 @@
     updatable: false,
 
     binaries: [
+        "compos_key_service",
         "compsvc",
         "compsvc_worker",
         "pvm_exec",