Send BUGREPORT_REQUESTED broadcast to system user

This broadcast should have a qualified user since
it is being sent from the system. This broadcast
is being sent to the shell, so we should use
UserHandle.SYSTEM.

Test: atest CtsBugreportTestCases, check logcat
      for warnings
Bug: 180769102
Change-Id: I3bebc886cf9931ea79fa0e91312d8720e5bd0e7d