Improve Recent Apps scrolling performance

Improve Recent Apps scrolling performance

- 20fps improvement using software rendering
- 10fps improvement using hardware rendering
- in sw mode, rendering recents background in the recent items themselves and using a bitmap cache to draw individual items (gives perf gains for sw mode)
- in sw and hw mode, no longer doing a fade on the recents scroll view (gives perf gains for hw mode) - instead we draw a black gradient where we would normally fade
- fading recents & notifications immediately when swiped
- removing unused code

Change-Id: I908e2a25b89c9dfbf9b8c8f3810fa43064261b33