Hide low-power mode black frame

Hide the wallpaper earlier and wait a little longer to
switch to low power mode.

This way the flickering area is less noticeable, since
less pixels will be visible, and users are less likely
to be looking at the screen.

Test: atest packages/SystemUI/tests/src/com/android/systemui/statusbar/phone/DozeParametersTest.java
Change-Id: I24f2d64f580d7fdc63f6c768446d06140c29cfa5
Fixes: 78302022