Fix the cts build.

ag/513580 had some problems. To fix:
-Removed use of "DEBUG" from MSSIMComparer. Its declaration was already removed
-Quick refactor of PerPixelBitmapVerifier to address a bad refactor
earlier. Functionality is hopefully is preserved, but I wasn't very
familiar with the code originally and can't verify this
on a device at the moment without access to my workstation.

Change-Id: Iace7c4f1c04143ccc91e0f79910e69bc3f771b8e