Csip: Add more means to get grouped devices

This change adds more convenient ways to get group members by group
identifier to ease the CSIP integration in HAP and possibly other
services, eg. BASS Client.

It also makes the grouped device list sorted from the lowest to
the highest rank, which is the order of CSIP Ordered Access
procedure.

Bug: 150670922
Bug: 230559809
Tag: #feature
Sponsor: jpawlowski@
Test: atest BluetoothInstrumentationTests
Change-Id: I765f4a82b5f3ff8d7644a86030846d79af8b6397