Ending "Paused" span for a fragment.

[magda-woj]

This may be a simple fix for the #590 issue I just submitted. However I don't know the reasoning for the current way, so I don't know if it creates new problems.

Currently onFragmentPaused and onFragmentStopped add events to the same span, which is ended in onFragmentStopped. I propose to end an active span in onFragmentPaused and create a new one in onFragmentStopped. With those changes Paused for About activity is a parent of Paused for the last fragment and analogously with Stopped (so 2 traces with 2 spans each) . Unrelated spans get their own traces in between and after.

In addition to fixing #590 this is consistent with how in Activity Lifecycle Instrumentation Paused and Stopped are separate spans.

[magda-woj]

(I've been meaning to add this before but forgot) I haven't changed the tests for this because I wanted to wait for a review of the idea not to change them needlessly, as I haven't dug into our unit tests. That's why it doesn't pass checks.

[LikeTheSalad]

Thanks!