InputDispatcher: Use constexpr values for debug flags

A simple refactor to use constexpr bool values instead of macros for
debug logs. This way, if when code changes are made, the compiler will
also check if the disabled debug statements compile.

This also moves InputDispatcher.cpp helpers and constants into an
anonymous namespace.

Bug: None
Test: atest inputflinger_tests
Change-Id: I1725ca2a03d3ad47a78dd15f2c660786d9f127c1