Add a lock around mBuffer in Archive.

Channel removal was resulting in some concurrent modification exceptions, so this change both adds a lock to synchronize any read/write access to mBuffer and also adds a test to make sure attempted concurrent access works correctly.

Test: atest ArchiveTest before and after change
Fixes: 186860793
Change-Id: Ic5f15a0fd3de27a44cc755c5d2f8008887c6b5b9