init: only set ro.boottime.<service> properties once.

Currently, init attempts to set ro.boottime.<service> properties
whenever a service starts, however since these properties are ro. this
means that an error is printed whenever a service is restarted.

Since these properties are intended for reporting boottime, these
subsequent writes during restarts are erroneous and therefore this
change stops attempting to write them, thus silencing the error.

Test: boot bullhead, restart processes, observe no error print
Change-Id: I372f8d5c26590fc0661b92f632410e23e6418841