Create composd skeleton

This has all the scaffolding/boilerplate to create a composd service
which can be started and serve a binder interface, but with no actual
implementation yet.

Bug: 186126194
Test: adb shell cmd -w android.system.composd; service starts.
Change-Id: I2ba91b36628b21210ac901525a93664473ac8460
diff --git a/compos/apex/Android.bp b/compos/apex/Android.bp
index 853b9f4..ada0976 100644
--- a/compos/apex/Android.bp
+++ b/compos/apex/Android.bp
@@ -40,6 +40,7 @@
     binaries: [
         "compos_key_cmd",
         "compos_verify_key",
+        "composd",
         "compsvc",
         "pvm_exec",
     ],
@@ -50,5 +51,13 @@
 
     prebuilts: [
         "CompOSPayloadApp.apk.idsig",
+        "com.android.compos.init.rc",
     ],
 }
+
+prebuilt_etc {
+    name: "com.android.compos.init.rc",
+    src: "composd.rc",
+    filename: "init.rc",
+    installable: false,
+}