Clean up VideoEditorPreviewController class part one:

o moved local types inside the class to avoid external exposure
o use non-virtual destructor since it is not meant to be a base class
o removed unused header files and moved some of the header files to .cpp file from .h file
o replaced #define constants with local enums

Change-Id: Ib321fc8c0379955083bc892c2ef93ed58c6c2ff5