dex2oat: Pack likely-dirty objects together when generating the boot image

dex2oat: Pack likely-dirty objects together when generating the boot image

This introduces a new algorithm into image writer which "bins" objects
by how likely they are to be dirtied at runtime. Objects in the same bin
are placed contiguously in memory (i.e. into the same page). We try to
tune the bin selection based on how clean or how dirty the object will
likely be at runtime.

As-is, this saves about 150KB per-process (private-dirty pages) and 700KB in
zygote (shared-dirty).

There is still about 800KB of objects that are clean but located in
dirty pages, so with more analysis we can tune the bin selection and get
even more memory savings.

Bug: 17611661
Change-Id: Ia1455e4c56ffd0a36ae2a723d35b7e06502980f7