set the extra_free_kbytes kernel vm tunable

Set the sys.sysctl.extra_free_kbytes property, which will proxy
through init to the /proc/sys/vm/extra_free_kbytes tunable (on
kernels where it exists).  This will ask the kernel to keep
more memory free, causing lowmemorykiller to fire earlier and
allocations to succeed faster.

The size is set to 3 full screen 32bpp buffers.  Products can
override the size with an absolute or adjustment value using
overlays config_extraFreeKbytesAbsolute and
config_extraFreeKbytesAdjust.

Bug: 10024467
Change-Id: Ib4d4507513ec3c1f4d4ceeb81ed632d1ad643437