Protect group from changing its integrity

This CL adds missing synchronized blocks where are operations on group.

Tag: #feature
Bug: 261362580
Test: atest LeAudioServiceTest
Change-Id: I5821d922cd4834e6b19e79d31df7f47f82eb8eef
Merged-In: I5821d922cd4834e6b19e79d31df7f47f82eb8eef
(cherry picked from commit 33b43ec2)