Fix grouped items not appearing if total items changed

We were originally only setting the flag isFirstDeviceInGroup if the
original list was empty. We should still set this value if the old list
was not empty.

Fixes: 398230187
Test: local test, atest
Flag: com.android.media.flags.enable_output_switcher_device_grouping
Change-Id: Iaa28a5bcf157f7a03363ed9cb56474a9719ee89e