Only compile dex files if they are not up to date.

Instead of trying to compile them all the time, and relying on file
permissions to keep us from overwriting up-to-date dex files with
different compiler filters.

Bug: 27641809
Bug: 28639246
Bug: 28826195
Change-Id: Ie717089d58517427b6f9a522b48146761c57a1a9
3 files changed