Taskbar in overview

- Behind ENABLE_TASKBAR_IN_OVERVIEW which is disabled by defualt
- Make taskbar unstashed in overview with background visible
- Ensure taskbar is touchable in overview

Bug: 245320601
Test: Home<->Overview, Home<->App, App<->Overview, Home->Overview->App->Home, Home->App->Overview->Home, Overview->Home->App->Overview, Overview->App->Home->Overview, App->Overview->Home->App, App->Home->Overview->App
Test: Go to AllApps, call KEYCODE_HOME, go to app and taskbar should be there
Test: App->Overview->Select->Home->App
Test: QuickSwitch -> App
Test: Repeat above with ENABLE_TASKBAR_IN_OVERVIEW x taskbarStashed on/off
Test: Repeat above with gesture and 3-button
Change-Id: Ib25d68f68e89564851b226536e456d0b2f6306f8