SF: Remove per-display state in scheduler

This CL removes per-display RefreshRateConfigs and AllowedDisplayConfigs
to avoid bugs in the untested multi-display code path of the scheduler,
adds checks to prevent crashes if the internal display is removed, and
cleans up related code by:

1) Replacing AllowedDisplayConfigs with a simple set.
2) Making setAllowedDisplayConfigs consistent with setPowerMode.
3) Removing unnecessary locking and allocation.

Bug: 129433906
Test: Boot with single/multiple display(s)
Change-Id: I3f59e9bdeaceb2cf48b4b9b71cd27f1d6a574680