feat(select): update the trigger width when select is opened

[emoralesb05]

When changing the width of the md-select (e.g. using flex box) after its rendered, the template[cdk-connected-overlay] does not update its minWidth and keeps the initial minWidth every time its opened. (somewhat the same issue as #3573)

This issue partially happens because the cdk-connected-overlay sets the width, minWidth, etc etc only once and never updates it again.

https://github.com/angular/material2/blob/master/src/lib/core/overlay/overlay-directives.ts#L181

But also because the _triggerWidth property in md-select is never updated either.

Here is a plnkr to demonstrate the issue: http://plnkr.co/edit/ERg71p?p=preview

Steps to reproduce

• Change the width of the md-select component by entering a diff value in the width input box under it. (both single and multiple md-select's will change)
• Open either md-select, and notice the width of the cdk-overlay-pane remained the same as when it was initially rendered.

Proposed fix in PR

We just need to set the new _triggerWidth every time the md-select is opened (open()) taking into account dir/offsetX and update the overlayRef#OverlayState in ConnectedOverlayDirective when the template is attached.

https://github.com/angular/material2/blob/master/src/lib/core/overlay/overlay-directives.ts#L188-L202

[emoralesb05]

@crisbeto not sure if this PR affects #3228 and #3247.. or the vision for how it should work

[kara]

@emoralesb05 Is this still an issue with the current version? If so, would you mind rebasing? Sorry for the late review.

[emoralesb05]

@kara yes its still an issue, i updated the plnkr with the latest version to try it.

Rebasing it so we can hopefully merge it soon 😄

[kara]

OK, ping me when it's ready for review again :)

[emoralesb05]

Im actually gonna create another PR since rebasing 5-6 months worth of code is taking its toll on me haha 😂 ..

[kara]

Haha, fair enough. Ping me when your new PR is ready, and I'll review ASAP to avoid more conflicts.

[emoralesb05]

@kara i opened #6489 so we can continue in that one 😄

Closing this one

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