LeAudioService: Put back @GuardedBy on mGroupDescriptors

https://r.android.com/2966700 remove the @GuardedBy annotation
on mGroupDescriptors. This is useful to catch bugs so put it back.

Bug: 325674688
Bug: 326295400
Test: None
Change-Id: Ib1ab83a85ba34bdfaadc69446b7c8a7a45b76250