Update user engaged sessions to handle multiple session per app case

- This is the first step for checking media notifications link with
media sessions for fullfilling requirements for app being in the
foreground.
- For simplicity, we are ignoring the cases of MediaSession2 as
Notification has no way to be linked with MediaSession2.

Bug: 295518668
Test: Manual playback
Change-Id: Ia62769215749e082c8cb7e2855852b2027f455d4