Bug 1072781 - [Window Management] Get screenshot for homescreen app since new getScreenshot API is ready, r=alive

- Remove the unnecessary homescreen checking and early return
Let's do one more check here to avoid performance impact.
if (System.isBusyLoading() && this.getBottomMostWindow().isHomescreen) {
  // We will eventually get screenshot when being requested from task manager.
  return;
}