import private keys from Bitcoin Wallet backup files

[johnzweng]

Adds a new button to the AddRecordActivity which allows the import of private keys from backup files created by the Schildbach "Bitcoin Wallet" app. Didn't create a new Activity but simply added AlertDialogs to AddRecordActivity (don't know if you'd prefer to have this in separate Activity).

Some details:

• button is not shown if no backup files exist
• works with testnet and prodnet
• allows to choose which file to import
• allows to choose which key from the file to import
• english and german strings included
• uses javax.crypto.Cipher class to decrypt the AES-256-CBC enciphered backups (uses no code from the Schildbach Bitcoin Wallet)