init: account for "init.svc." prefix in IsValidName

For each service ${service} described in a .rc file, init creates the
property init.svc.${service}.  This implies that service-name lengths
are bound by the maximum size of a property.  This patch accounts for
the "init.svc." prefix and the maximum property-name length when
validating a service name.

Test: pass

Change-Id: I32dffa37363b41a8211f7644600b81973d30a3b2
Signed-off-by: Iliyan Malchev <malchev@google.com>