Add signing command for testing.

Extend the AIDL to allow for signing. This will need to be backed out
before shipping, but it's needed for testing right now.

Add a command to sign a set of files to compos_key_cmd & write the
signature files.

Move compos_key_cmd from virt to compos APEX. (We're definitely going
to need this code in CompOS in some form, so might as well make sure
all the dependencies are available.)

Bug: 190166662
Test: Manual: Sign artifacts, check odsign accepts them.
Change-Id: I25361ed0bee52a9ff13924c77d9378efe8bfd314
diff --git a/compos/apex/Android.bp b/compos/apex/Android.bp
index 1fffa2e..2dded99 100644
--- a/compos/apex/Android.bp
+++ b/compos/apex/Android.bp
@@ -37,6 +37,7 @@
     platform_apis: true,
 
     binaries: [
+        "compos_key_cmd",
         "compos_key_service",
         "compsvc",
         "compsvc_worker",