Use group icon in device chip for group routing sessions

This change extends ag/27342016 by also overriding the icon when the
number of selected routes is greater than one. In which case we use the
group icon.

Note: This issue was not caused by ag/27342016. But it's also unclear
when it was introduced.

Bug: b/288915598
Test: Manually using music playing apps.
Flag: com.android.settingslib.media.flags.use_playback_info_for_routing_controls
Change-Id: Ic7a15362f7ed51ee1b1da40ee4c30722d1e7f7b3