Add RIP20 support to Android RVN Wallet

[TronBlack]

This Ravencoin Improvement Proposal #20 (RIP20) will give a number to the user in addition to their 12-words. The number will be the number of weeks since Ravencoin Genesis block.

Add the extra number when displaying 12-word backup information (the number should not be given or should be 0 if you don't know when the seed was generated).
Accept the OPTIONAL number when restoring the wallet.
Jump to the date when syncing to speed up the sync on a wallet restore.

https://blog.ravencoin.email/post/12-words-and-a-number

[TronBlack]

amount=USD$2000.00
paid_in=RVN
status=in progress
info=https://github.com/RavencoinFoundation/gitRVN