MIDI: fix building of properties for the native side

A couple of MIDI device's properties (related to ALSA) are of int type
and can't be transferred as a string. Fixed the code.

As we whitelist properties that can go into native, there is no need to
expect other type of properties besides ints and strings.

Bug: 25945784
Change-Id: I008731d0ffee60c31be0bfddb15dcd1425e4b88e
Test: using a PoC app