Limit the sample length to interval on the activity graph when there are no samples after/before

In the activity graph, samples extend more than they should when there are no samples before or after that one.
For example:

There are no samples in the area that is marked, as you can see in the sample graph below. So, this are should left blank (or have another indicator), so people can actually see the that there is no activity in that area. Currently it looks like it's previous and next samples are looking a lot longer than they actually are.

So as a solution, we should limit the sample length to intervals. This is not necessarily correct either but at least it's more accurate compared to the current visualization

[Example profile](https://deploy-preview-3098--perf-html.netlify.app/public/rvh0n155hj416h23abtfynzzxyhabsyx2rqnzk0/calltree/?globalTrackOrder=0-1-2-3-4-5-6-7-8-9-10-11-12&localTrackOrderByPid=96008-13-14-0-1-2-3-4-5-6-7-8-9-10-11-12~96010-0~96021-0~96020-0~96011-0~96019-1-0~96017-2-0-1~96014-1-2-0~96018-1-0~96015-1-2-0~96016-1-2-0~96013-3-4-0-1-2~96012-1-2-0~&range=3637m756&thread=17&timelineType=cpu-category&v=5)

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-364)