chore: improve type safety in breakpoint observer

[devversion]

Fixes a workaround where we cast the MediaQueryList event listener to any because TypeScript didn't have proper types before TS 3.1.

[josephperrott]

LGTM

[mmalerba]

I get the following error when trying to presubmit this:

breakpoints-observer.ts:116:7 - error TS2345: Argument of type 'MonoTypeOperatorFunction<MediaQueryList>' is not assignable to parameter of type 'OperatorFunction<MediaQueryListEvent, MediaQueryList>'.
  Types of parameters 'source' and 'source' are incompatible.
    Type 'Observable<MediaQueryListEvent>' is not assignable to type 'Observable<MediaQueryList>'.
      Type 'MediaQueryListEvent' is not assignable to type 'MediaQueryList'.
        Property 'onchange' is missing in type 'MediaQueryListEvent'.

116       startWith(mql),

[devversion]

@mmalerba I will have a look. Let's remove merge ready in the meantime.

[devversion]

@mmalerba I think I've fixed that. Could you re-run the presubmit if you find some time? Thanks!

[jelbourn]

@devversion could you rebase this?

[devversion]

@jelbourn Done. I was also able to remove another cast to any since we now use a TypeScript
version that has proper types.

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