Allow microdroid_manager to stop tombstoned

If export_tombstones is false, leaving tombstoned running has no
meaning. However, we still can't selectively start tombstoned, because
post-fs-data happens eariler than config parsing. Thus, this change
allows microdroid_manager to stop tombstoned on demand.

Bug: 236588647
Test: atest MicrodroidTests
Change-Id: I813fe667f3394bdd234e204f3d35a27f3a182cb2
diff --git a/microdroid/system/private/property.te b/microdroid/system/private/property.te
index 28fb8e1..a02a7f2 100644
--- a/microdroid/system/private/property.te
+++ b/microdroid/system/private/property.te
@@ -1,3 +1,5 @@
+system_internal_prop(ctl_tombstoned_prop)
+
 system_restricted_prop(boot_status_prop)
 
 # Declare ART properties for CompOS