Don't access system properties from vendor code.

A recently introduced optimization reduces the service polling
interval to 100ms during boot. For this it uses the boot completed
system property. However, vendor code isn't allowed to access
system properties. This change keeps the behavior as it was,
while not accessing the system prop. We can investigate later
if we can do something similar for vendor code.

Bug: 80153956
Test: builds, sailfish boots
Change-Id: Ic52039516ae3e912ff2ec7c8dced4951696678df