Smarter image layout

Put strings in the dex file that resolves them.

Depth first traversal with overrides for class and dex cache. The
work list keeps track of what oat_index with each pushed item. This
means the static fields of a class will usually be in the same image.

Added layout test to image_test to make sure things are somewhat
reasonably attributed.

Bug: 28640955

Test: test-art-host

Change-Id: I67a536c33aeed603b252d8e0f75622c9efbf2559