SF: Avoid registering DisplayDevice with Scheduler

The Scheduler should only care about the RefreshRateSelector part, and
should not needlessly extend the compositionengine::Display's lifetime
until the DisplayDevice is unregistered.

Make Scheduler::registerDisplay infallible, such that SurfaceFlinger::
processDisplayChanged does not need to unregister before registering.

Bug: 241285191
Test: libsurfaceflinger_unittest
Change-Id: I12b3855167e98f48ae368d39264edcb456efb293