Store odex files in oat/<isa>/ directory.

Previously odex files were stored alongside the dex location as:
  dex location: /foo/bar/base.apk
  odex location: /foo/bar/<isa>/base.odex

This changes where odex files are stored, adding an "oat" directory:
  dex location: /foo/bar/base.apk
  odex location: /foo/bar/oat/<isa>/base.odex

See also the corresponding change in platform/art and
platform/frameworks/native.

Bug: 19550105
Change-Id: I4c6be4f0c41ff175904846db8e360c4af815b265