fix(select): improve screen reader support

move to WAI-ARIA's Collapsible Dropdown Listbox practice
only set `aria-checked` for selects with `multiple`
- manually remove `aria-checked` set by ngAria due to ngValue usage
apply `md-focused` class to the option with focus
implement `aria-activedescendant` and `aria-multiselectable`
improve `ng-multiple` implementation
- account for `multiple` attribute on `md-select-menu`
- put `aria-multiselectable` on the correct element
remove unused `deregisterCollectionWatch()`
fix overloaded variable names

Fixes #10748. Fixes #10967.

Author: Splaktar