[IP Protection] Fix OnIpProtectionEnabledChanged Flaky Test

This CL works to re-enable and fix the OnIpProtectionEnabledChanged test
by adding a flush after setting IpProtection to enabled, and changing
the flush when disabling IP Protection to flush the main network
interface.

The current behavior of the test is flaky when checking whether the
state of IP Protection was changed after being disabled. The flakiness
could most likely be due to not waiting for the response long enough
that IP Protection's status was changed. By adding a flush to the
remotes for the IPProtectionProxyDelegates, we wait for the
`ip_protection_core_impl::CoreControl` message to be processed before
checking for if IP Protection was enabled / disabled. This should reduce
the flakiness of the tests by ensuring the settings are properly set
before checking.

Bug: 445760641
Change-Id: I610d911afd5a1488ab31432f74491b97f1232e4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7003219
Commit-Queue: Antonio Alphonse <alphonsea@google.com>
Reviewed-by: Andrew Williams <awillia@chromium.org>
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1527134}