AppWindow: performance and memory improvements

- create views when showing the AppWindow and mNeedsUpdate is true
- don't recreate views with each app install/update/removal
- remove all views before creating a new instance

Change-Id: Iedf573b77787bacf88d6d0a4a2ff09568dfb43f0
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>