fix(FocusZone): Respect defaultTabbable element when FocusZone container gets focus

[sophieH29]

defaultTabbableElement should be taken into account, when shouldFocusInnerElementWhenReceivedFocus is true and focus goes to FocusZone container.

Here is a decision tree:

• Focus goes to FocusZone container (usually when it's an inner zone of other FocusZone) and shouldFocusInnerElementWhenReceivedFocus === "true"
• If defaultTabbableElement is provided by client, try to fetch element and focus it
• If no - focus first focusable element

[miroslavstastny]

• rename this test in FocusZone-test.tsx:

it('should force focus to first focusable element when FocusZone container receives focus and shouldFocusFirstElementWhenReceivedFocus is set to "true"', () => {

• update CHANGELOG