Bug 1071451: [AUM] Handle attention windows properly so calls are counted

It makes me nervous to declare new variables within a single case of a larger switch statement like this. As a reviewer, I can't tell from the patch alone whether this is safe or not because I don't know if these variables are already in use.  I'd suggest doing this without the variables:
```
if (this.attentionWindows.length) {
  this.metrics.recordInvocation(this.getLastAttentionWindow().manifestURL,
          now - this.getLastAttentionWindow().startTime)
}
else {
  this.metrics.recordInvocation(this.currentApp, now - this.currentAppStartTime);
}
```