android/lowmemorykiller: Wait for memory to be freed

The memory reclaim code needs to give time to the system to
return the memory from a killed process otherwise the memory
reclaim code could run continuously, in multiple threads,
which could starve both the watchdog thread and the thread
which is responsible for returning the memory from the
killed process.

Change-Id: Ib390dbb021cb6d41e74ac64eca6fef46367daf1e
CRs-fixed: 447740
Signed-off-by: Liam Mark <lmark@codeaurora.org>