fix: sort field items according to dex_file_verifier.cc

[eybisi]

registerClass creates wrong dex file when there are more than 2 fields #296

This commit fixes two errors:

• field_ids items are not sorted.
• field_idx_diff wrongly calculated. (0,1,2,3) needs to be (0,1,1,1)

Since we create 1 dex file for each registerClass, we don't need to keep track of field_idx_diff. (okay to begin from 0)

[oleavr]

Thanks! :raised_hands: