Refactor DexViewBuilder

The code was previously pretty messy and hard to follow. This reworks some
things to make it more manageable. Among the changes:

* Pull out fragments of code into their own method, so the high level steps are
  more apparent in methods like DexViewBuilder::Start.
* Pull frequently used types into global constants in dex_layout_compiler.cc.
* Rework register handling to track liveness so a strict stack discipline is no
  longer needed.

Change-Id: Idb4b41f88c96a1ac4efb2c7b9bed05c2de0da999