FETCH_SYNC_DEVICES should return only devices

[ayumi]

Fix #75

Auditors: @diracdeltas

Test plan:
0. First confirm it's broken on master:
a. Start browser-laptop with a new profile.
b. Enable Sync
c. Visit archive.org and disable shields.
d. Visit wikipedia.org and disable shields.
e. Restart Brave.
f. Console shows 3 devices / 2 too many:

sync 1492637047413: fetching BOOKMARKS,PREFERENCES records after 1492637031
sync 1492637047761: fetched 3 devices
sync 1492637047871: fetched 3 PREFERENCES after 1492637031

g. Restart Brave.
h. Console shows again 3 devices:

sync 1492637059167: fetching devices
sync 1492637059196: fetching BOOKMARKS,PREFERENCES records after 1492637047
sync 1492637059559: fetched 3 devices

i. Close Brave.

1. Go to brave/sync, checkout this branch fix/list-devices and yarn dist.
2. Restart Brave on the existing profile.
3. Console should look like:

sync 1492637080118: fetching devices
sync 1492637080144: fetching BOOKMARKS,PREFERENCES records after 1492637059
sync 1492637080503: fetched 1 devices
sync 1492637080504: got 0 decrypted records in PREFERENCES after 1492637059
sync 1492637080504: getting existing objects for 0 PREFERENCES
sync 1492637080505: got 0 decrypted records in BOOKMARKS after 1492637059
sync 1492637080505: getting existing objects for 0 BOOKMARKS