Remove mPendingCallback from ActivityTracker

Instead, all callers should use EXTRA_SCHEDULER_CALLBACK, set via
addToIntent(), to provide a callback for the next handleIntent().
The updated runCallbackWhenActivityExists() will do that for the
caller if the activity doesn't already exist (otherwise it will
run the callback synchronously as before).

Bug: 151389129
Change-Id: Idbec264354fd6de166ff3bae98249230078d5674