bug(expansion): toggle icon not being updated

[julianobrasil]

Bug, feature request, or proposal:

bug

What is the expected behavior?

If you switch the expanded input from false-true-false... the toggle icon on the upper right corner should change it's direction.

What is the current behavior?

It's not changing, unless you click on the expansion header

What are the steps to reproduce?

At the plunker, click on the toggle panel button bellow the expansion to switch its state and watch the not-changing toggle icon.

Plunker: https://plnkr.co/edit/SIkVCu?p=preview

What is the use-case or motivation for changing an existing behavior?

Programmatically open and close the expansion

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Material latest builds

Is there anything else we should know?

expanded == true

expanded == false

[crisbeto]

Picking this one up as per chat with @devversion.