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: Ieded4bfe038ca936247fa4b638070e979b02eaa1
CRs-fixed: 447740
Signed-off-by: Liam Mark <lmark@codeaurora.org>