java: persistent data

Currently persistent data is stored in *.txt files in the root folder and read at startup.
Encryption key for exchange with phone: key.txt (in clear)
Apple Notification Service Token: apsToken.txt (in clear)

The already existing persistent data mechanism of Bisq should be used.

Start by locating the startup code that reads the root-level *.txt files, especially key.txt and apsToken.txt, and then find Bisq's existing persistent data mechanism. Trace how the encryption key and Apple Notification Service token are stored and loaded. Done means these values no longer use clear-text root files and the existing Bisq persistence mechanism is used.

Written by the indexing model from the issue text.