Put MultiTouchMotionAccumulator in its own files

This will let us use it in the new TouchpadInputMapper.

The change mostly just moves the code into new files, but also makes a
few other small cleanups:

* Directly includes stuff the code depends on, rather than indirectly
  through EventHub.h
* Renames private variables of Slot to capitalize acronyms consistently
  and in line with Google C++ style (i.e. any case of "AbsMT" replaced
  with "AbsMt")
* Rewraps a comment in ::configure
* Uses ALOGW_IF to avoid another nested if block in ::process
* Put all the initializers on one line in the constructor, to satisfy
  clang-format

The inline was also removed from Slot::getToolType, due to compile
errors complaining that it wasn't defined otherwise.

Bug: 251196347
Test: atest inputflinger_tests
Test: manual on device
Change-Id: I538178a2222904c5bab5622d257d7747842358a7