uid_sys_stats: defer io stats calulation for dead tasks

Store sum of dead task io stats in uid_entry and defer uid io
calulation until next uid proc stat change or dumpsys.

Bug: 37754877
Change-Id: I970f010a4c841c5ca26d0efc7e027414c3c952e0
Signed-off-by: Jin Qian <jinqian@google.com>
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>