No need to consider invalid tracks when checking if all active tracks are rerouted.

When a track is invalid, it will be removed later. In that case, there
is no need to consider such tracks when checking if all active tracks
will be rerouted.

Bug: 175343099
Test: connect/disconnect BT when playing music
Change-Id: I074b861af1f59affaf21b4ae5614512265464e1d