Turn on and off hw vsyncs for secondary display

When the power mode changes on non-"active" displays, still turn on/off
hw VSYNC callbacks. The call to turn them on is crucial - when a display
is turned off, not only are hw VSYNCs turned off, they are disallowed.
It is only by calls to resyncToHardwareVsyncLocked that they are again
allowed. Without this change, this would have to wait for a call to
resync or kernelIdleTimerCallback. This improves predictions for the
secondary display.

When turning off the active display, if there is an activatable display,
still call disableHardwareVsync for the old active display.

Bug: 255601557
Test: SetPowerModeInternalTest and FoldableTest
Change-Id: Ida666af9a3a9b2e940c1f861ce3b765f67738f1f