Backport of extra_free_kbytes to Jellybean

The value is calculated based on the screen height/width in 4.4:
width * height * 4 * 3 / 1024

We set:
320x480 (MDPI): 1800
480x854 (HDPI): 4804

Thanks to Conn O'Griofa <connogriofa@gmail.com>

Change-Id: I644a4c870a66e8ca12f036afbe890b508d589d16