Set dex_cache_strings_ when we call Class::SetDexCache

Ensures that these two variables never get out of sync. The error
was presumably related to not doing this for proxy classes. This
caused java code which was looking at the dex_cache_strings_ field
to incorrectly access a null array.

Bug: 18548887

(cherry picked from commit ea1c3d77)

Change-Id: I022d9311b38b61e160ed70e3c5d9639797adb29c