WidgetTray improvement

UI
- Fixing the issue of widget row not rendering
- Moved the resetting of previous bitmap to the start of onBind
instead of the onDetachedWindow.

Performance
- WidgetModel do not have to keep packageName list around.

Will com in immediate follow up CLs
- use lowRes widgets bitmap before fully loading the fullRes one.
- application icons are fully loaded in background thread.

b/19897708

Change-Id: I5b3b43a5c543a5135c395f90df32e66d57dc86e0