libkmpris: always update active player index on rows inserted
THe active player index can change on rows inserted, if:
- Play a video in a browser with pbi
- Pause the video
- Start another player B
- Play B to make it become the active player
- Pause B, and B is still the active player
- 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.
- 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