Make sync settings less confusing.

b/10602459
It was possible to turn off syncing for an account in global
settings, but we'd continue displaying some sync frequency
in the in-app settings.
Now, we only display the sync frequency if sync is actually
enabled. If it's disabled we always display "never". Also,
when the user changes a sync setting, if it's set to "never",
we leave the frequency in the database as it is, but disable
sync for that account. If it's set to anything else, we store
that in the database and ensure that sync for the account is
enabled. This means we should not have any apparent disagreement
between in-app settings and global settings as to whether or
not syncing will happen.

Change-Id: I1cc54e76aafd25dc4db0f1b713e7d7cbc30bf77f
1 file changed