Use current directory to load prediction model

Before this CL, the testing binary could not execute properly when
invoked directly. This is because the current working directory does not
get set.

To fix this, use the model file from /system/etc when __ANDROID__ is
defined, and refer to the current directory for all other cases.

Test: m libinput_tests && $ANDROID_HOST_OUT/nativetest64/libinput_tests/libinput_tests
Test: atest libinput_tests
Test: atest --host libinput_tests
Bug: 271134652
Change-Id: I0547ca9666eab153208a581fb60f6bef656b946a