Use backgroundColor for the old toast layout.

Some history necessary to understand why this is needed:
- From Android R, we started rendering text toasts in System UI instead
  of having them rendered by the app. As a result, we have two separate
  layouts for toasts:
  * targetSdk < 30: transient_notification.xml (rendered by the app)
  * targetSdg >= 30: text_toast.xml (rendered by SystemUI)
- We previously used backgroundColor for the toast background
(toastFrameBackground), but we moved to using surfaceColor (a system
color which matches the device theme).

The problem is that surfaceColor isn't available when the toast is
rendered by the app though, leading to weird behaviors like showing
light grey text on a white background if the app's theme is material
dark.

To fix this, we should still use backgroundColor (like before) for
toasts that are rendered by the app.

Fix: 245678793
Test: manually triggered toast from app targeting SDK 29, observed
color is correct

Change-Id: I7ab1991c628b85f01391e41b6a03b3bd7f219058

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