microdroid_manager: initial impl

It is started by init in microdroid and executes a command specified in
a VM payload config.

Bug: 189301496
Test: MicrodroidHostTestCases
  (in a microdroid, run /system/bin/microdroid_manager manulally)
Change-Id: I85c7e370d4a0dcf58b4aafbe6e9fba73e69c2a44
diff --git a/microdroid/init.rc b/microdroid/init.rc
index 121afaa..8458291 100644
--- a/microdroid/init.rc
+++ b/microdroid/init.rc
@@ -110,6 +110,9 @@
 
     perform_apex_config
 
+    # TODO(b/185991357) start this earlier
+    start microdroid_manager
+
     exec_start derive_sdk
 
     start adbd