Improve AyncTask throughput inside WidgetPreviewLoader

b/21133230
> Synchronized block was creating a bottleneck for the AsyncTasks.
> Remove calls that doesn't need to be synchronized outside synchronized block.
> Also removed setAlpha call as after the bottleneck was removed,
Inefficient alpha view usage alert started popping up in traceview.
Due to less jankness, removing the fadein animation doesn't have any
visible effect.

Link to lock congestion visualization:
https://x20web.corp.google.com/~hyunyoungs/no_crawl/traceview/traceview_lockcontention.html

Result: gfx-avg-jank delta = "-1"
Change-Id: If12817df0730f346cdba7e2f38f232eb9a4336c0