[1/2] [WIP] Camera: add software HDR rendering

This is still very experimental - while it works, software HDR is
force-enabled so far.

This allows devices without HDR camera (so all phones except Nexus 4
I think...) to actually take HDR pictures. When the mode is enabled,
pressing the shutter button will take 3 shots, one a minimal exposure,
one at 0 exposure, and one at max exposure. The 3 shots are then fed
into a software HDR processor that will merge the 3 images into one
final shot.

There are two algorithms available for merging pictures, one is a
simple average between all pixels, and the second one is based on
Reinhard's algorithm. They all need tweaking, and only the first one
is used for now.
Average algorithm is available in RenderScript form. This beastie
speeds up a lot!

Todo list:
- Enable HDR toggle in the UI
- Freeze UI while shots are taken
- Show a progress bar while the final HDR shot is being rendered
- Find a better way to determine when the sensor has set the exposure
  (takes between 1 and 2 seconds on an i9300, some older phones are
  faster).
- Generate a real name for the HDR output file

Change-Id: I7a9e52e56258a0c755f4def22db51ac84501ce9a