[1/2] Camera: add software HDR rendering

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!

Source shots are removed after HDR is computed

Change-Id: I7a9e52e56258a0c755f4def22db51ac84501ce9a