groups: Add debug dump

Adds the following info to the dumpsys logs:

Device Groups Manager:
  Num. registered clients: 2
  Groups:
    == Group id: 1 ==
      Uuid: 0000ffff-0000-1000-8000-00805f9b34fb
      Devices:
        XX:XX:XX:XX:XX:XX
        YY:YY:YY:YY:YY:YY
    == Group id: 2 ==
      Uuid: ...
      ...

Bug: 150670922
Tag: #feature
Test: atest --host bluetooth_groups_test
Sponsor: jpawlowski@
Change-Id: Ie3306462ec15f39123647324a3fb0b90e7686ec7