Enabling users to dismiss the heap dump notification.

1. setOngoing meant that users were not allowed to dismiss the
notification, even though a deleteIntent was added to the notification.

2. showWhen is by default set to false, so calling setWhen had no effect

3. Trying to delete the heap dump accidentally reposted the
notification.

Bug: 127581643
Test: manual
Change-Id: Ifbdb3c9800756b28656bc8175ef8f90902c397c7