backends: Avoid orientation changes around suspend/resume if locked

We currently lock the capability of the MetaOrientationManager to emit the ::orientation-changed signal, but otherwise keep reading the current orientation and returning it if we are asked politely through meta_orientation_manager_get_orientation().

This may bring issues around suspend/resume, since there may be other parts of the code trying to get the current orientation without receiving ::orientation-changed signals.

In order to fix this, make the MetaOrientationManager return a fixed orientation while locked, only updated after changes in the lock state.