Use the correct object to guard |mConversationsListeners|

The variable is marked as @GuardedBy("mLock"), but actually guarded
by itself.

Bug: 276714246
Test: m
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:cae843c3878eff70ae223c3e22b19558cb594b04)
Merged-In: I383e6c42f16f747dd80d604671b369fa491376cb
Change-Id: I383e6c42f16f747dd80d604671b369fa491376cb