Add the finishing task as a trigger for crash.

When activity crashes, set the finishing task as the trigger task. This
allows shell to better handle crashes through #handleRequest.

Fix: 361058348
Fix: 356187158
Test: Open any app, crash it using "adb shel am crash", see that trigger
task set instead of being null.
Flag: EXEMPT Bugfix

Change-Id: I24d63ad5e74a336871327872d007663d65f8cbfc