fix(material-experimental/mdc-list): make internal input non-interactive

Due to how MDC's checkbox styles are set up, we always have to keep an `input` inside
the `mat-list-option`, however our current setup where we only set `tabindex="-1"` on it
is invalid, because the input is still interactive either through clicking on it, or by using
a screen reader's forms mode.

These changes completely remove the input from the accessibility model by setting it
to `display: none`.

Author: crisbeto

src/material-experimental/mdc-list/list-option.html

@@ -15,9 +15,9 @@
       no accidental change event leaks out of the list option or selection list when
       the checkbox is directly clicked.
     -->
-    <input type="checkbox" tabindex="-1" class="mdc-checkbox__native-control"
-           [checked]="selected" [disabled]="disabled" [attr.aria-describedby]="_optionTextId"
-           (change)="$event.stopPropagation()" />
+    <input type="checkbox" class="mdc-checkbox__native-control"
+           [checked]="selected" [disabled]="disabled"
+           (change)="$event.stopPropagation()"/>
     <div class="mdc-checkbox__background">
       <svg class="mdc-checkbox__checkmark"
            viewBox="0 0 24 24">
@@ -41,7 +41,7 @@
 </ng-template>
 
 <!-- Text -->
-<span class="mdc-list-item__text" #text [id]="_optionTextId">
+<span class="mdc-list-item__text" #text>
   <ng-content></ng-content>
 </span>
 

src/material-experimental/mdc-list/list-option.scss

@@ -4,3 +4,13 @@
 // The MDC-based list-option uses the MDC checkbox for the selection indicators.
 // We need to ensure that the checkbox styles are included for the list-option.
 @include mdc-checkbox-without-ripple($query: $mat-base-styles-query);
+
+// The internal checkbox is purely decorative, but because it's an `input`, the user can still
+// focus it by tabbing or clicking. Furthermore, `mat-list-option` has the `option` role which
+// doesn't allow a nested `input`. We use `display: none` both to remove it from the tab order
+// and to prevent focus from reaching it through the screen reader's forms mode. Ideally we'd
+// remove the `input` completely, but we can't because MDC uses a `:checked` selector to
+// toggle the selected styles.
+.mat-mdc-list-option .mdc-checkbox__native-control {
+  display: none;
+}

src/material-experimental/mdc-list/list-option.ts

@@ -51,9 +51,6 @@ export interface SelectionList extends MatListBase {
   _onTouched: () => void;
 }
 
-/** Unique id for created list options. */
-let uniqueId = 0;
-
 @Component({
   selector: 'mat-list-option',
   exportAs: 'matListOption',
@@ -89,9 +86,6 @@ export class MatListOption extends MatListItemBase implements ListOption, OnInit
   @ContentChildren(MatLine, {read: ElementRef, descendants: true}) lines:
     QueryList<ElementRef<Element>>;
 
-  /** Unique id for the text. Used for describing the underlying checkbox input. */
-  _optionTextId: string = `mat-mdc-list-option-text-${uniqueId++}`;
-
   /** Whether the label should appear before or after the checkbox. Defaults to 'after' */
   @Input() checkboxPosition: MatListOptionCheckboxPosition = 'after';