Keep original order of fields in Class.

The fields of a class are ordered alphabetically in the dex
file. Keep the same order in the field arrays so that we can
do binary search lookups by name. Those lookups will be
implemented in a subsequent change in libcore/.

Bug: 18211592
Change-Id: I8f979de62ffe37d1c7d5c721717d2f3501e7c9e6