Fix remote wipe keychain storage (issue #1592)

Fix remote wipe keychain storage (issue #1592)

The app password for the remote wipe was constantly being written in
WebFlowCredentials::slotFinished to the keychain, leading to unnecessary
write and log overhead on the system.

This fix introduces a check to only store the app password once in
a lifetime of the Account class. Also the method used to store the
password will be renamed from setAppPassword to writeAppPasswordOnce
to be more expressive.

Signed-off-by: Michael Schuster <michael@schuster.ms>