Camera2: Allow devices to opt-out of threaded camera startup

On at least the current Exynos based devices, the vendor's HAL
appears to be unable to handle the threaded startup.  Since we
cannot easily fix their HAL, allow these devices to opt out
of the faster camera startup.

Change-Id: I8d1935eb69bb8ba1396b59d92993062637654258
Merge remote-tracking branch 'cm/cm-11.0' into HEAD