ActivityManagerService: Add an overall stack dumping timeout.

An ANR can result in stack dumps of a generally unbounded number
of processes. Dumping occurs on a thread that's patrolled by the
watchdog, so we should make sure that all dumps trigger within a
specified timeout.

Test: Manual; by setting the timeout to 1s.
Bug: 37177331

Change-Id: I626272dfbcc9f8b57c1ef1321fac43cdb937a885