Check whether doze is enabled before starting the screen off animation.

We should always have been doing this, but it was masked by the WM color-fade screen off animation and was only visible in certain situations (such as AOD enabled but suppressed by power saver).

Fixes: 192380279
Test: screen off with/without power saver
Change-Id: I88e3479173cfd32675d45a2801e5ab7518b4d8b0