init: expand arguments of services when they start

Arguments of commands are expanded based on properties if they contain
${property.name}, however this is not currently done for arguments of
services.

This patch makes it that arguments of services are expanded each time
that the service starts at the point immediately before execve().

Bug: 28788401
Change-Id: Iba581a8377e25a6478d4d2ec2e8b29e181d8640c