Camera: SoftwareHDR: Reduce mem usage and let exposure time get tuned

- The memory usage has been reduced. Instead of doing 3 bitmaps and
  keeping them in memory, only one bitmap is used and recycled once
  copied to the renderscript allocation.
  In counterpart, I had to remove the software (non-RS) processing.
  I haven't seen any issue of devices disliking RenderScript, so
  I think it's fine.

- I added a config overlay to set the exposure settle time between
  the shots. Some devices are faster than others (ZSL devices can
  take less than 1 second, the previous value). I set this to 0.8
  seconds by default, which works fine on i9100/n7000.

Change-Id: Ia067b717a3754ffb423b29a915d2ae411d04f0b9