TouchInputMapper: move PointerDistanceHeapElement definition

This struct is only used in one method, so declare it there instead of
in the header file. Also take the chance to rename the distance field to
be more descriptive.

Test: $ atest --host inputflinger_tests
Flag: EXEMPT refactor
Bug: 245989146
Change-Id: I4001c5b1bac9d913bd16a091f9f3b28165928fca