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 would like to move condition out of the do-something function, like to make it as '_checkAbleToAddTopMask'. But since this is a simple function, so be it.