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.
Merge pull request #24285 from mnjul/bug_1060541_lckscr_noti_scrolling_hvga_for_1038723

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