refactor(select): Use pre-existing single-selection list logic.

Also fix list foundation so that tabindex is set correctly (closes #6143)

PiperOrigin-RevId: 318506841

Author: allan-chen

packages/select/src/mwc-select-base.ts

@@ -578,6 +578,8 @@ export abstract class SelectBase extends FormElement {
       },
       addClassAtIndex: () => undefined,
       removeClassAtIndex: () => undefined,
+      getSelectedIndex: () => this.index,
+      setSelectedIndex: () => undefined,
       isTypeaheadInProgress: () =>
           typeahead.isTypingInProgress(this.typeaheadState),
       typeaheadMatchItem: (nextChar, startingIndex) => {