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This cl sets "playStoreMode" policy to restrict
child user from toggling Play Store account.

This cl updates "applications" policy's value for
playStore to also restrict the child user from
toggling Play Store account.

Both values have to be set because there is a race
condition that exists between the two policies.

The policies are updated from the client side and not
from the server side because the client has enough information
to set the values. In addition, sending the additional policy
values from the server will unnecessarily incur bandwidth costs
overtime (few bytes sent per policy update per child adds
up overtime).

Test: The CL has Unit tests to cover the changes. It has also been
manually tested on device.

Bug: 1106901
Change-Id: I56d95d955c8fe54b84ea7c94cabea302d7d8378f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2304996Reviewed-by: Polina Bondarenko <pbond@chromium.org>
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Commit-Queue: Yilkal Abe <yilkal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792727}