Fix max touch count calculation in TouchpadInputMapper

It seems I got confused between InputDeviceContext.hasKeyCode and
hasScanCode.

Bug: 251196347
Test: check that max_touch_cnt in the HardwareProperties is set
      correctly (e.g. with the tests from ag/20691427)
Change-Id: Ief9144deb90360e1f12f35316b174a40de8c3db4