floss: Add Appearance field to bt_property list

Floss needs this field from the EIR in order to emulate how metrics
work in BlueZ. Therefore, store the appearance into storage, and pass
it with the callback for Floss only.

Bug: 240782193
Tag: #floss
Test: Verify Appearance field in /var/lib/bluetooth/bt_config.conf
      after pairing with an LE mouse.

Change-Id: Iac4f2fea7f1f4f711b3f99a7d416799831b1f948