Don't turn on prox until after screen is off/dozed

This change ensures that the doze related code does not turn on the
proximity sensor until after it has completed its transition to a low
power screen state.

This fixes the appearance of a white dot on devices with the proximity
sensor behind the screen.

Fixes: 155441239
Test: atest SystemUITests && manual
Change-Id: I58cc9a8e92fa0b219ada13c293478c6fc5e79c78