[13/n Dispatcher refactor] Move TouchState to DispatcherTouchState

This CL moves mTouchStatesByDisplay to DispatcherTouchState subclass as
a public attribute. Upcoming CLs will remove direct access to it and
eventually make it private.

Bug: 367661487
Bug: 245989146
Test: atest inputflinger_tests
Flag: EXEMPT refactor
Change-Id: I4db90c626abb909e685eb06c8a8ec82d22a572b9