Divide InputDispatcher into several files.

This CL does:
1) Isolate implementation details of InputDispatcher from outside and
only expose necessary header files;
2) Move implementation details into android::inputdispatcher namespace;
3) Make input dispatcher a static library for inputflinger to link
against;
4) Add InputDispatcherFactory.{h,cpp} to isolate implementation details
in InputDispatcher.h from InputManager.

Bug: 140139676
Test: Touches on touchscreen can be dispatched to right windows.
Change-Id: Ib61c16fd41f3f76f538a3de9b54f31ac304e03a5