Move the markers back to the top of activity graph in active tab view

Previously, in our design discussions, we've decided to move the markers to the bottom of the activity graph. But right now, we also have sample graph there and it's really making it hard to see which one is a marker and which one belongs to samples graph. Moving markers back to the top of the activity graph fixes this problem.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-520)