Look for prediction model in vendor partition

When loading the prediction model, we should first check the vendor
partition, and then, if that doesn't work, use the system one. This will
allow OEMs to customize this model.

Bug: 210158587
Test: loaded model into /vendor and deleted from /system, checked that
prediction works

Bug: 271455682
Test: atest libinput_tests inputflinger_tests
Change-Id: I0a369e5ec5cec8ac20b66fb4fcf265e7b1dde38a