Permissions: Fix account related permissions.

Requires updating the docs in AccountManaager as well as the logic in
AccountManagerService.

MANAGE_ACCOUNTS, USE_CREDENTIALS, and AUTHENTCATE_ACCOUNTS are going
away. Where AUTHENTCATE_ACCOUNTS was required we now do signature
matching.

GET_ACCOUNTS is kept but has been grouped under contacts.

Bug: 20136477
Change-Id: Iabbb76dce8d1efc607c1f107911d7ddab598a481