use loaded notification polices instead of query
use loaded notification polices instead of query

we preload all the notification policies, so we want to use find instead
of a query to check for fallback channels

test plan
 - specs should pass

refs VICE-1280
flag = none

Change-Id: I963610d6cf76236e4a69f33f52932694c9751984
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/263325
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Omar Soto-Fortuño <2ca100155bc0669318a7e6f3850629c7667465e1@instructure.com>
QA-Review: Omar Soto-Fortuño <2ca100155bc0669318a7e6f3850629c7667465e1@instructure.com>
Product-Review: Omar Soto-Fortuño <2ca100155bc0669318a7e6f3850629c7667465e1@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms