Revert "ActivityTrigger:  New class to invoke when activity starts/resumes"

 * No longer supported upstream, and will be superseded by new
   CM functionality.

This reverts commit c5bb1bbd0a59640ec394ee729b57de34295a4e99.

Change-Id: Ieb26fb1db62b739e048a6a9b87fb33579b4ee285