Delegate existence of account check to Authenticator.

Current AccountManager code for getAuthToken checks if the account
in the request exists. If the account does not exist then it throws
an exception which leads to a runtime exception being thrown by
AccountManager in the client. In perticular, Checkin client code
hits this issue when accounts are deleted by user. As the exception
is thrown from the getAuthToken method call and is a RuntimeException
it is not caught by the client. Futhermore, Checkin runs in one of the
important processes and this exception makes the process crash.

This cl, does the following:
1) Delegates the account exists check to Authentictor which in turn
would cause an AuthenticatorException which is a checked exception.
2) Replaces some of the runtime exceptions thrown by AccountManagerService
with calling AccountManagerResponse.onError() which causes more graceful
failure on the client.
3) Correctly passes on the error returned by Authenticator to
AccountManager. Earlier if Authenticator returned an error code to
the AccountManager, it ignored the error and returned null token to the
client which was incorrect.

Bug: 10856295
Change-Id: Ie250fec601d46f6dfecd74677b478bfd4e9dcfad