Split compos_key_service.

Split the code that implements the service from the code that starts
it as a binder service. This is preparation for also allowing it to be
started in a VM.

Bug: 193603140
Test: Run compos_key_host, connect to it via compos_key_cmd
Change-Id: I8f223ef19c9490dc349e1f677f670eedcf09dead
diff --git a/compos/Android.bp b/compos/Android.bp
index 1eb6716..ba270b8 100644
--- a/compos/Android.bp
+++ b/compos/Android.bp
@@ -67,8 +67,8 @@
 }
 
 rust_binary {
-    name: "compos_key_service",
-    srcs: ["src/compos_key_service.rs"],
+    name: "compos_key_host",
+    srcs: ["src/compos_key_host_main.rs"],
     edition: "2018",
     rustlibs: [
         "compos_aidl_interface-rust",