8243596: ModuleLayer::parents should return an unmodifiable list

[Alan Bateman]

This is an small oversight and inconsistency that crept in when the 
ModuleLayer was updated to support multiple parents. The list of parent 
layers returned by the parents() method should be specified (and 
implemented) to return an unmodifiable list. It currently returns a 
modifiable list. The equivalent Configuration::parents is correctly 
specified, as is the Configuration::modules and ModuleLayer::modules 
methods.

The change proposed here fixes the javadoc and implementation. The 
compatibility impact is minimal as nothing could depend on being able to 
dynamically change the list of parents (as it can't be changed dynamically).

I've extended the existing tests to ensure that ::modules, 
Configuration::parents, ModuleLayer::modules and ModuleLayer::parents 
return unmodifiable collections. In passing, there is also a typo in the 
Configuration::modules where the javadoc says "immutable" instead of 
"unmodifiable".

http://cr.openjdk.java.net/~alanb/8243596/webrev/

The patch doesn't update the date in the copyright headers, I'll update 
those before pushing after the changes (and CSR) are agreed.

-Alan