set background property for the layout used in makeCustomToastWithIcon()

Set android:background property in the layout used in custom icon toasts
to match with the layout of the systemui toast.

Screenshots:
go/ss/AcaRQUJQfh79wDw.png
go/ss/5j4oD8xydwX7HTX.png
go/ss/7y9BguWfYBnng7y.png

Bug:330176222
Test: Verify toast with custom icon has round corners
Change-Id: I686b71d76278bb44b7c4dd3b830bd4bd14889d07