Don't use java.nio channel for file size determination

Don't use java.nio channel for file size determination

Java NIO has some problems (like files closing unexpectedly because the
thread was interrupted). To avoid those problems, don't use a NIO
channel to determine the size of a file, but rather ask the File itself.

We have to be prepared to handle wrong/outdated information in this case
too, as the inode of the File may change between opening and determining
file size.

Change-Id: Ic7aa6c3337480879efcce4a3058b548cd0e2cef0