libkmpris: always update active player index on rows inserted

THe active player index can change on rows inserted, if:

1. Play a video in a browser with pbi
2. Pause the video
3. Start another player B
4. Play B to make it become the active player
5. Pause B, and B is still the active player
6. Close the browser. The pbi quits first, invalidating the filter
   model, and the original browser instance is added back, which is
   inserted before the current active player B.
7. An assertion is triggered because the active player index does not
   match the active player.

This also adds a test to consolidate the fix.

BUG: 483027
FIXED-IN: 6.0.2


(cherry picked from commit 78b1142f)