SurfaceFlinger: lock mStateLock when modifying allowed configs

mAllowedDisplayConfigs should be protected by mStateLock when changing
it from the main thread in setAllowedDisplayConfigsInternal() as it is
being read by Binder threads in getAllowedDisplayConfigs().

Test: call setAllowedDisplayConfigs() and getAllowedDisplayConfigs()
      from backdoor using different threads.
Bug: 142850581

Change-Id: I569e75968aea676c96fbc648140ac49a165ffb69
Merged-In: I569e75968aea676c96fbc648140ac49a165ffb69