AI 145870: Add a build property for the default alarm alert. Update the various

  framework classes to deal with the new property. Also update various
  documentation that mentions the default ringtones.
  Use the build property as the default alert when the user has not chosen
  an alert for an alarm. This is also used if the alarm alert is null when
  the alarm fires.
  BUG=1723684

Automated import of CL 145870