Use mmap to read TFLite model.

The buffers in the model file are used directly by TFLite, and so a
small memory saving can be achieved by backing those memory pages with
the file itself.

Bug: 267050081
Test: atest libinput_tests
Change-Id: I743a3c94477d4bb778b6e0c4b4890a44f4e19aa4