RFR: 8248131: Simplify ServicesCatalog provider handling

[Claes Redestad]

Hi,

the current implementation of ServicesCatalog uses an internal providers
method, which is only used to add ServiceProviders.

Providing an addProviders method instead means we can streamline this,
to the tune of a very small startup win.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8248131
Webrev: http://cr.openjdk.java.net/~redestad/8248131/open.00/

Testing: tier1

Thanks!

/Claes