Workspace: Custom default screen

Change-Id: Ic90254f7758b9dbd4997a19474e455230355b772
Trebuchet : Fix search bar icon/text disapearing

Repro :

1. Uninstall an application from the app drawer
2. Press back to go to homescreen.
3. The search bar icon/text should be invisible now.

Trebuchet : Avoid unecessary generatePressedFocusedStates

When the drawable's state changes, don't generate state
drawables until after the invalidate call, which takes care of this
already, otherwise it gets drawn before measure/layout gets a chance to run.

Change-Id: I49933e6d985aab9fff9f05092e007e46991a8edf