WidgetTray performance improvement (less sorting): Part 1

- No longer return sorted list from the package manager since, the only
time the widget list requires to be sorted is when recycler view renders
them.
- Made getWidgetsAndShortcuts private to guarantee that widgets are
being loaded in the worker thread.

Change-Id: I2c35973c1226e831514380dd38fc2f88b1b91d02