OMAP3 has 3 layers, OMAP5 has 4.
You only need those to show four buffers at the same time (three of which support scaling and color space transformation), with individual placement on the display, z-order, and alpha compositing. By default one layer is used, attached to the buffer belonging to /dev/fb0, but you can happily detach it without the application using it even realizing it's now drawing in an off-screen buffer. Or you can use one of the other layers for an overlay.
Isn't that what we want?
One Overlay reserved fo notifications.
A Popup that opens up when someone calls or you get a message.
Then the user can decide what to do.