API: naming: rename Strings -> InteractionModelBuiltins or similar

[mikelid-zz]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[ ] Feature request
[ ] Documentation issue or request
[x] API improvement
[ ] Other... Please describe:

ask-sdk-controls exports a namespace called 'Strings' which contains enumerations for standard / built-in interaction model things: Targets, Actions, Feedback. The name should reflect this. Also, it should be obvious that the actual enumeration values are 'builtin_xyz' to help with comprehension.

However, a good name is hard to find!