Do not unregister lift when prox is covered

The lift sensor won't receive all the data that it needs otherwise.

Test: atest DozeParametersTest
Test: lift while prox is covered
Test: lift while prox is covered, uncovering before end of gesture
Test: lift while prox is not covered
Fixes: 134685251
Change-Id: I0938baba6ccaaf04c40fe30b3f4f59293179dc2a