Switch preffered display mode on lock screen

To save power, now it's possible to specify a preferred refresh rate for
the lock screen. It's -1 by default, and should be configured on a
device overlay whenever required.

Test: manual
Test: atest StatusBarWindowControllerTest
Fixes: 138303350
Change-Id: I4d1ca210b50d32d937b0fea2fcb9e28a7c50c0b3