WidgetPreviewLoader concurrency issue / Caching improvement

1) Concurrency issue: unused bitmap was not properly synchronized
which caused concurrency issue. Hence, leading current widget tray
implementation to not use it. (a.k.a. cancel(false)).
Issue fixed and now using the unused bitmap pool.

2) Caching improvement: LoadedBitmap cache was a legacy support system
for the old widget tray implementation. On our latest implementation,
cache and recycled view is completely being managed by the recycler view.
Hence removed.

Change-Id: I843e6a286b676f283172f2c1ef5cbeed0a9fb17f