android/lowmemorykiller: Check all tasks for death pending

The lowmemorykiller uses the TIF_MEMDIE flag to help ensure it doesn't
kill another task until the memory from the previously killed task has
been returned to the system.

However the lowmemorykiller does not currently look at tasks who do not
have a tasks->mm, but just because a process doesn't have a tasks->mm
does not mean that the task's memory has been fully returned to the
system yet.

In order to prevent the lowmemorykiller from unnecessarily killing
multiple applications in a row the lowmemorykiller has been changed to
ensure that previous killed tasks are no longer in the process list
before attempting to kill another task.

Change-Id: I7d8a8fd39ca5625e6448ed2efebfb621f6e93845
Signed-off-by: Liam Mark <lmark@codeaurora.org>