Make sure tombstone is transmitted before VM exit

This change makes tombstone_transmit more robust by doing:
1) running tombstone_transmit *after* mounting APEXes, because
   tombstone_transmit isn't a bootstrap binary.
2) marking tombstone_transmit as shutdown critical, for init to wait
   tombstone_transmit upon shutdown.
3) stopping tombstoned, in case export_tombstones is false.

Bug: 236588647
Test: atest MicrodroidTests MicrodroidHostTestCases
Change-Id: I53f93ed384bdc0e558e0d899a0c9e7b01623a933
diff --git a/microdroid/init.rc b/microdroid/init.rc
index be08bbd..b0e5e46 100644
--- a/microdroid/init.rc
+++ b/microdroid/init.rc
@@ -179,6 +179,7 @@
 service tombstone_transmit /system/bin/tombstone_transmit.microdroid -cid 2 -port 2000 -remove_tombstones_after_transmitting
     user root
     group system
+    shutdown critical
 
 service apexd-vm /system/bin/apexd --vm
     user root