[init] trigger late_start on property change

Merge counterpart: https://github.com/mer-hybris/droid-system-packager/pull/1

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`