Extract display size monitor

Detecting display size changes is not straightforward:
 - from a DisplayListener, "display changed" events are received, but
   this does not imply that the size has changed (it must be checked);
 - on Android 14 (see e26bdb07a21493d096ea5c8cfd870fc5a3f015dc),
   "display changed" events are not received on some versions, so as a
   fallback, a RotationWatcher and a DisplayFoldListener are registered,
   but unregistered as soon as a "display changed" event is actually
   received, which means that the problem is fixed.

Extract a "display size monitor" to share the code between screen
capture and virtual display capture.

PR #5455 <https://github.com/Genymobile/scrcpy/pull/5455>