Callbacks for progress and connection state listeners holds references to sessions that are not released when removing the listeners. 

This was introduced for connection listeners in https://github.com/realm/realm-java/pull/6884/files#diff-8c785429417eedf1dd260bab03dafcb1R258

but similar issue exist for progress listener.

Address leftover review comment from: https://github.com/realm/realm-java/pull/6884:

We should probably rename the java_syncmanager_class variables as that is no longer accurate