Created a new bug report workflow so user can keep track of its progress.

The old workflow was:

1. dumpstate starts.
2. When dumpstate finishes, it sends a BUGREPORT_FINISHED intent.
3. Shell's BugreportReceiver receives the BUGREPORT_FINISHED and issues a
   system notification so user can share the bug report.

The new workflow is:

1. When dumpstate starts, it sends a BUGREPORT_STARTED with its pid and
   the estimated total effort.
2. When Shell's BugreportReceiver receives the BUGREPORT_STARTED, it:
  2.1 Issues a system notification so user can watch the
      progresss (which is 0% initially).
  2.2 Starts a service (BugreportProgressService) responsible for
      polling the dumpstate progress (using system properties and the
      pid) and updating the system notification.
3. As dumpstate progress, it updates the proper system property.
4. When dumpstate finishes, it sends a BUGREPORT_FINISHED event.
5. When Shell's BugreportReceiver receives the BUGREPORT_FINISHED, it:
  5.1 Finishes the service if necessary.
  5.2 Issues a system notification so user can share the bug report.

This CL handles the Shell changes only, the dumpstate changes will be
changed in a separate CL.

BUG: 25794470
Change-Id: Icbd0b42dd48e8db376b60544348b6818c6374338