Fixes dumpstate crashed when shutdown the thread pool

The future of the packaged_task should be cleared before we
clear the packaged_task in the task queue.

Bug: 182798355
Test: atest DumpPoolTest#ShutdownBeforeTasksFinished_withoutCrash
Change-Id: I85e6aca67c61629d786f0586c6fe3d714f400233
2 files changed