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/microdroid_file_contexts b/microdroid/microdroid_file_contexts
index b033110..fc5e456 100644
--- a/microdroid/microdroid_file_contexts
+++ b/microdroid/microdroid_file_contexts
@@ -368,6 +368,7 @@
 /system/bin/snapuserd            u:object_r:snapuserd_exec:s0
 /system/bin/zipfuse              u:object_r:zipfuse_exec:s0
 /system/bin/microdroid_launcher  u:object_r:microdroid_launcher_exec:s0
+/system/bin/microdroid_manager   u:object_r:microdroid_manager_exec:s0
 
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 # Vendor files