staging: android: lowmemorykiller: select a new task to kill

Under certain circumstances, a process may take time to handle a SIGKILL.
When lowmemkiller is called again shortly after, it would pick the same
process to kill over and over, so that we cann't get free memory for long time.

Solution is to check fatal_signal_pending() on the selected task, and if it's
already pending, select a new task to kill.