Simplify handling process died for activity

WindowProcessController should maintain the activity list belonging
to it, so there is no need to go through all stacks to match the
died process. Now it is simpler and more efficient by just iterating
the activities of the process directly.

Bug: 161447610
Test: RootWindowContainerTests#testForceStopPackage
      ActivityRecordTests#testRemoveFromHistory
      ActivityStackTests
Change-Id: I21a647408046bbf7d52813996c263bc992a1deb8