Changed dvmDexCacheStatus to check for odex file first.

This prevents the generation of a new dexfile when an odex file already
exists when switching between Art and Dalvik.

Bug: 13463154
Change-Id: I55ef9abe1fe172ee656550e78556381f95698c33
Merge branch 'kitkat-dev' of https://android.googlesource.com/platform/dalvik into kitkat-dev