Ignore prox data when it's obsolete

The proximity sensor response is asynchronous, this means
that the user might have interacted with the phone while
sensor data is being processed. Let's ignore its response
in case it arrives late.

Fixes: 129012983
Test: press power while uncovering prox sensor
Change-Id: Ib31e6c53b9f5993349c5eaed6122af5228d07728