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: I611af172c6b7d8f8a639f9602af37d710e8a1340