staging: android: lowmemorykiller: Don't count reserved free memory

The amount of reserved memory varies between devices. Subtract it
here to reduce the amount of devices specific tuning needed for the
minfree values.

Signed-off-by: Arve Hjønnevåg <arve@android.com>
Change-Id: I466ae8b18f5972f6f6d8b5a7d8c4ae69660de53a