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/aidl/Android.bp b/compos/aidl/Android.bp
index 07bec09..4d36d3d 100644
--- a/compos/aidl/Android.bp
+++ b/compos/aidl/Android.bp
@@ -17,7 +17,7 @@
         },
         ndk: {
             apex_available: [
-                "com.android.virt",
+                "com.android.compos",
             ],
         },
     },