Fix flaky TextClassifierSettingsTest.

We used to override and read DeviceConfig directly, but it is temporary and will be override after the testing emulator syncs. With this change, we always override/read a fake DeviceConfig.

Bug: 191844721
Bug: 204903521
Change-Id: I06021adc2e075c1745e1027fda85235d61957702
(cherry picked from commit 5c9980a957e85c74e6577f5ebe300a4a5039b742)