Lazily allocate observables

[cramforce]

E.g.

amphtml/src/input.js

Line 75 in bf39315

this.touchDetectedObservable_ = new Observable();

We should optimize memory usage for cases where never anything registers, or were registration is rare or non-immediate.

[cramforce]

I think the best win would be to avoid the array allocation here https://github.com/ampproject/amphtml/blob/master/src/observable.js#L28 so that unused observables only cost one object.