Increase communication surface between dumpstate and Shell:

- Add a new 'dumpstate' context for system properties. This context
  will be used to share state between dumpstate and Shell. For example,
  as dumpstate progresses, it will update a system property, which Shell
  will use to display the progress in the UI as a system
  notification. The user could also rename the bugreport file, in which
  case Shell would use another system property to communicate such
  change to dumpstate.
- Allow Shell to call 'ctl.bugreport stop' so the same system
  notification can be used to stop dumpstate.

BUG: 25794470

Change-Id: I74b80bda07292a91358f2eea9eb8444caabc5895