Reduce number of physical pages used for DexCache's arrays

This commit reduces the amount of physical pages allocated for the .bss
section of an application's oat file.
Many of elements in the application's DexCache arrays are empty.
But during coping content of the DexCaches arrays from
the app's art file into the .bss section all pages become dirty.
The commit fixes that by copying non null elements only. So pages
in the .bss section that contains zeroes remains untouched.
This approach allows to save 209Kb of memory after one minute of
GMaps use.
This number is the difference between the size of .bss section and
the Rss value from the /proc/<pid>/smaps file.

Test: run test-art-host

Change-Id: I3d7bed0805b95b5f344303581274ca8e8e69940e