transfer.py: Move Transferer, MariaDB logic and Firewall logic to its new module files

Handling MariaDB and handling the Firewall are two logically separate units.
So, move MariaDB related methods to its new MariaDB class and firewall related
methods to its new Firewall class. Further, move Transferer class to its
new file Transferer.py.
This separation reduces the length of the transfer.py file.

Bug: T252172
Change-Id: I1974ff801bed8902e351378fa5aed79627f5f54c