Add functions to Jobs to retrieve current progress

There is no change in behaviour as this patch just adds functions.

Due to limitations in the current job implementations, there are a few
shortcominings:

* Failure reasons aren't exposed.
* Transfers of zero bytes (e.g. folder only or empty file) show as
  indeterminate progress.
* In cases where file size can't be determined, the job keeps track via
  files processed, but these will show as indeterminate progress.

Bug: 385841721
Test: atest -c 'DocumentsUIGoogleTests:com.android.documentsui.services'
Flag: com.android.documentsui.flags.visual_signals

Change-Id: I7b0d863bf7b974090c0e1a17f06c0f7160c59699