Fix Disallow config Wi-Fi message for AFW Do mode.

Correctly displays the custom message set by the Test DPC tool
for devices in AFW Do mode.

This change also fixes an issue where when Wi-Fi is disabled, the
disabled message is not shown instantly on Settings app start.

Bug: 37134344
Bug: 37911257
Test: runtest --path
packages/apps/Settings/tests/unit/src/com/android/settings/wifi/WifiSettingsUiTest.java
for off message testing, manual inspection using the Test DPC tool, and
verifying Guest mode behavior.

Change-Id: I61e48f605d75cb5665461536beeedcb32f6aa7f9