Fix potential race condition in Call.

1) Add getConnections method to Call and return a copied mConnection field.
2) Add removeConnection, addConnection, getConnectionsFrom methods to
Call to avoid direct access of mConnection.
3) Add synchronization of all operation on mConnection.

Test: atest FrameworksTelephonyTests.
Bug: 131876970

Change-Id: If8f8df48ad52226cea401a82dfc22f6edb1f607e