chore: make all event emitters / subjects readonly

[rafaelss95]

Fixes #9582.

[jelbourn]

LGTM

[mmalerba]

I'm marking this for patch release even though it's technically breaking, nobody should be setting those though, I hope...

[rafaelss95]

@jelbourn not really related to this, but while I was doing this PR, I noticed that were some EventEmitter's that uses next instead of emit.

The cases are:

https://github.com/angular/material2/blob/7613f98a7b73711f4f67957d5f135700c41ebd7e/src/lib/paginator/paginator.ts#L193

https://github.com/angular/material2/blob/7613f98a7b73711f4f67957d5f135700c41ebd7e/src/lib/sort/sort.ts#L131

The problem with that is that it was deprecated on PR @angular/angular#5302.

---

On the other hand, this works perfectly because EventEmitter extends Subject and emit calls next (of Subject) behind the scenes:

https://github.com/angular/angular/blob/3aaf43f73c17570f61d3d0cc672a01695b716499/packages/core/src/event_emitter.ts#L78

In a nutshell, I'm not sure if there's a problem calling next instead of emit, but I thought it's good to tell you.

[jelbourn]

Not a problem, but feel free to send a PR if you want

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