Don't create a giant layer for all notifications

Bug #6642475

When expanding the status bar, create one layer per notification instead of
a single giant layer for the pile of notifications. This prevents layer
creation failure when the total height of the notifications is larger
than the maximum allowed texture size in OpenGL ES 2.0.

This change only enables layers on notifications that will be visible
once the notification area is fully expanded.

Change-Id: I3c791a66cf5ac0973f3a65cfcd84b95209d580f3