[LE Unicast] Using the active device as main device after grouping

After onGroupIdChanged, the CsipDeviceManager assigns the first
device with the same groupId as main device. It is not correct
since the main device shouble be the active device. This issue
causes the summary can't shows 'Active' at the Settings.

Bug: 232892046
Test: Build pass
Change-Id: I00e5471b9315c896b8c16723ad37d66a922c27e4