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.

(cherry picked from commit 36e356c96640775f0a3f167bd2426ea0f0093b8b)

Change-Id: Ieafdca8453f70c96edb787b5a3a3ac7c7aeea021
Merge "Changed dvmDexCacheStatus to check for odex file first."
Merge https://android.googlesource.com/platform/dalvik into aosp-staging