chromeos: Replace ModifyCacheState with a number of small functions to simplify DriveCache
ModifyCacheState was doing a number of things at the same time:
- Move or copy a file. (and copy was only used by Store())
- If the symlink_path is not empty, create or delete symlink
That says, ModifyCacheState is unnecessarily complicated so that it can be dividend into four small functions, MoveFile, CopyFile, CreateSymlink and DeleteSymlink.