Add strings for Parental Controls Settings and Add Supervision Dialog

Note:  The code that uses these strings is in a different WIP CL that
will follow this one.

Bug: 927957
Change-Id: Ia77dc06bd4fe4ecb8125360a259fe3571b9c0756
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623532
Commit-Queue: Danan S <danan@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662464}