Typeaheadcontroller typeahead wrongly activated on menu navigation

Nobody has claimed this yet.

### What is affected?

Component, Accessibility

### Description

Putting focusable components - such as a `md-checkbox` - inside a `md-menu-item` and subsequently pressing 'TAB' when focused on said menu-item, correctly delegates focus to its inner checkbox, but also deactivates the parent `md-menu-item` - e.g sets `tabIndex = 0`.
Further navigation - e.g pressing `ArrowUp` - which is handled through a call to the `menu.listController.handleKeydown` function, will conclude that no items are active and therefore the call to `activatePreviousItem` with `activeItemRecord` being a `null`-value results in a call to `activateLastItem`.
The last list item gets focus, instead of the item above.

The culprit is the `menu.typeaheadController` whose `onKeydown`-function is called through the capturing event listener `menu.captureKeydown`. As `isTypingAhead == false` `beginTypeahead` gets called.

There are checks for whether or not the given key is a navigable key - upon which it should return - but only for arrow keys, space, enter and escape - hardcoded. Therefore shift and tabkeys begin typeahead's which sets `tabIndex = -1` for the `lastActiveItemRecord` (the first menu-item with `tabIndex == 0`);

### Reproduction

The below code (partially from the documentation) for a menu and activating button should display the described behaviour, when tabbing while focus is on the 'banana' `md-menu-item`, and then trying to navigate afterwards

```
Set with idref

```

### Workaround

Including more checks for which keys are navigable/non-typeahead;
Could just add shiftLeft and tab to the long if-statement, or maybe provide an extension point like the `listController`'s `isNavigableKey` function :) ;

### Is this a regression?

No or unsure. This never worked, or I haven't tried before.

### Affected versions

All (i imagine)

### Browser/OS/Node environment

unrelated i believe

Start at the menu typeaheadController.onKeydown path invoked by menu.captureKeydown, then trace listController.handleKeydown, activatePreviousItem, and beginTypeahead. Add a regression test for Tab navigation from a menu item containing a checkbox, and verify that subsequent ArrowUp navigation focuses the preceding item rather than the last item.

Written by the indexing model from the issue text.