Fix bug where accounts are duplicated in user&account list.

This happens when user has different accounts with same account name. For
example: user@domain.com for type1 and same email for type2, etc. When
we refresh the account list during onResume(), the same account name
incorrectly causes the logic to think they are the same account and
updates the account list by mistake.

- Introduced a util method to generate unique key from Account object.
And set it as preference key.
- when updating preference on screen, use key to find preference instead
of the preference instance itself.

Change-Id: I0aa692cb965b7037155a746389a919cd155843da
Fix: 34035653
Test: make RunSettingsRoboTests