[8/n InputDispatcher refactor] move input connections to a subclass

In this CL we create a new subclass for DispatcherInputConnections in
Dispatcher to consolidate functionality related to input connections.

Bug: 367661487
Bug: 245989146
Test: atest inputflinger_tests
Flag: EXEMPT refactor
Change-Id: I222df2770bdcd04f4c9a5985dc9651681e1d9fa4