Prevent recomputeDisableFlags from clobbering the system disable flags

- The disable() call on the binder thread can race with this call on
  the main thread due to this 1) fetching the old values on the main
  thread, then 2) trying to synchronize on mLock by calling disable()
  (which the binder thread is currently holding) and then 3) applying
  the previous flags immediately after the fresh disable flags are set
  by the binder call.

Bug: 205921118
Test: atest SystemUITests
Change-Id: I5dd4d2d248b6c1ab6d1677e2bb915100cd3e4f4e