Make sync settings restore more robust

Bug: 18506992
Parent Bug: 17967106
Introduce a new state for ContentResolver#getIsSyncable.
This state specifies that an adapter should be disabled until
explicitly turned on by ContentResolver#setSyncAutomatically(true).
In this way we can restore disabled sync adapters and still allow
them to run their initialization logic later on when they are
re-enabled.

Change-Id: I03fd1f994c4bc982bbc723154ba20bb252efdf80