madvise away app image dex cache arrays

This saves ram and PSS by reducing the number of mapped pages. Also
PROT_NONE the dex cache arrays section to make accesses fault.

virtual                     shared   shared  private  private
size      RSS      PSS    clean    dirty    clean    dirty     swap
-------- -------- -------- -------- -------- -------- -------- --------
Before:
/data/app/com.facebook.katana-2/oat/arm/base.art
52484    52144    43522    17244        0        0    34900        0
/data/app/com.google.android.apps.maps-2/oat/arm/base.art
 6876     6816     6816        0        0     2008     4808        0

After:
/data/app/com.facebook.katana-2/oat/arm/base.art
52484    49204    42032    14344        0        0    34860        0
/data/app/com.google.android.apps.maps-2/oat/arm/base.art
 6876     6388     6388        0        0     1584     4804        0

Bug: 22858531

Change-Id: I87888a7c1518a92736dbd29eed338e7472ae3587