MatSelect accessibility rework

This issue tracks rework of the MatSelect component to resolve multiple accessibility issues. Based on this prototype (just the a11y behavior and API, not the visuals)

Summary:

• New template-based syntax for options driven by data instead of ngFor (the existing syntax will remain for backwards compatibility for at least two major versions)
• Option text will always be in the DOM, but not the option templates
• Interaction changes from listbox to combobox

This should capture issues reported in #5220, #10292, #6382, #3587, #1670

[jelbourn]

@crisbeto has been looking into it. The initial attempt at fixing this broke a huge number of unit tests inside Google, so we’re trying another approach that will be more opt-in.

[jelbourn]

We’re now planning to address the issues as part of integrating the MDC Web components into Angular Material. The earlier attempts at fixing this were all just too much of a breaking change, such that it was impossible to roll out to applications in Google already using MatSelect.