Refractor the code for imgdiff

Put ImageChunk and some helper functions into a class. Also switch to
using std::vector instead of malloc.

Bug: 18606652
Test: imgdiff_test passed on host. Also generate a complete incremental OTA package.
The file content is the same and time consumption is similar.

Change-Id: Id603ada4e130ef521218400761a119001a86ca79