Bug 1060541 - Add 1 additional notification to lockscreen with new music player by bug 1038723

- Also, remove the top-mask (implemented in bug 1023500) when the visually top notification in the notifications container viewport is being actioned upon.
nit: I know this is hard to explain (as we discussed) and only use as a local variable, but I would add some ASCII diagram here. Let me try to done this with:
     
        - container -          = viewport =
    ------------------------==============------
    |   0  |  1  |  2  |  3  |  4  |  5  |  6  |  7  |  8  |
    |       |      |      |      |  0  |  1  |  2  |  3  |  -   |
    ------------------------==============-----
    The top most notification ID in container is `#4`, and the last one is `#7`
    This variable would keep the number `4` since in viewport we have 4 notifications.

I think this may be helpful, no matter whether we rename the variable from the original version (they're isomorphic).