[init] trigger late_start on property change

Android's late_start is triggered by mount_all, which also determines whether
the /data partition is encrypted.

Mer rootfs now has to spawn a service after mount points are ready, and before
peripheral services (such as sensors, wlan, bt, ...) are started. That service
has to issue `setprop droid.late_start trigger_late_start`

rootdir/init.rc

@@ -360,9 +360,14 @@ on boot
     class_start core
     class_start main
 
+# Never gets called, since Mer does its own 'mount_all'
 on nonencrypted
     class_start late_start
 
+# Mer needs to set this property when fs units are mounted
+on property:droid.late_start=trigger_late_start
+    class_start late_start
+
 on charger
     class_start charger