evdev: Multitouch code style changes.

1. Move touch count reporting helpers to utils. They are not multitouch.
2. Use evdev_mt prefix for private multitouch support routines.
3. Use int instead of int32_t where fixed size is not required.
4. Export some internal functions.

This change should be no-op.

(cherry picked from commit 98a7606b85e05132f328a2498dccab78df31cb7e)