[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

Hint: devices with native HDR support (like mako) must need to set
the overlay value which is introduced here from false -> true

Change-Id: I7a9e52e56258a0c755f4def22db51ac84501ce9a
Merge "[1/2] Camera: add software HDR rendering" into jb4.2