fix(cdk/a11y): activeItem out of date if active index is removed from ListKeyManager

[crisbeto]

Fixes the activeItem on the ListKeyManager not matching the item at the activeItemIndex, if the activeItem is removed from the list.

Fixes #14345.

Note: this is a resubmit of #14407.

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[mmalerba]

Caretaker note: caused change after checked errors in one test in g3

[yuriyward]

@crisbeto Hi, can you explain why ListKeyManager keeps the old activeItemIndex when it's not present in an array of new values for QueryList? Why this index is not set as -1? (from test list-key-manager.spec.ts:96 it looks as desired behavior)

Update: I found this PR, probably it will be changed soon. It causes a lot of problems when it's used together with virtual-scroll and searching functionality

[andrewseguin]

I was able to get a passing test suite with these changes:

          if (newIndex !== this._activeItemIndex) {
            setTimeout(() => {
              this.setActiveItem(
                  newIndex > -1 ? newIndex : this._activeItemIndex);
            }, 0);

1. Use setTimeout to avoid changed after check errors
2. Call setActiveItem for clients depending on the this.change.next event output

[andrewseguin]

Rolling back because it caused real issues in Google, not clear why yet

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