Fix TaskbarBackgroundRenderer not being applied correctly in 2 cases

- Rename isInAppAndNotStashed to areTaskbarIconsVisibleAndNotStashing,
  and check taskbarViewController.areIconsVisible() accordingly. This
  effectively handles the same as before, but also handles Overview,
  which is not inApp but does have areIconsVisible()
  - This fixes bubble scrim over overview taskbar, as well as showing
    taskbar background behind assistant in 3 button mode.
- Also fix nav bar button dark density changing when applying the
  separate background for assistant

Test: Open bubble and invoke assistant in overview, over IME, inside
an app, over home
Fixes: 266715337

Change-Id: Ie655de7abd1634e2165543d57664d7c7e054a986