Using the number of copied docs to calculate completed when required bytes is zero.

Fix notification will show "NaN %" when copy empty directory or empty file.

Also refector getProgressNotification() & speed estimation part to track
copy progress by ProgressTracker.

Change-Id: I62f104ef6b4391ab0cd7d5b3257ab09c2ca8da24
Fixes: 29616419
Test: atest DocumentsUITests:CopyJobTest#testCopyEmptyDir
Test: atest DocumentsUITests:CopyJobTest#testCopyProgressWithFileCount
Test: atest DocumentsUITests:CopyJobTest#testCopyProgressWithByteCount
Test: Manual, make sure notification progress & speed estimation
      work for Copy / Move / Compress job.