[FEATURE] Create an AccountInterface (Revision 1)

This change adds an AccountInterface which the SecuritySystem can rely on
instead of requiring a fixed Accout class. This is useful if you want to use
a completely different provider + storage where you can't inherit from the account.

Change-Id: I32a5805d84ed019d53a64672be679fbe5b791a7b
Revision-Id: 0eb6131680ca7548a6dfcd0e872d665ac4acb26c