Allow to use ExtService as TextClassifierService

1. Define a trust TextClassifierService list which contains ExtService. And
also define a DeviceConfig flag which can be set to the TextClassifierService
we would like to use. Because the TextClassfierService can be set to a side-load
app, e.g. CTS tests. If the override flag is not one of the packages in the
trust TextClassifierService list, it should be able to see only text from
its own UID.
2. Made TextClassifierService available for testing.
3. Fix the incoreect order for onConnected().
4. Fix API Lint errors.

Bug: 141111813
Bug: 143249163
Test: atest FrameworksCoreTests:android.view.textclassifier
Test: Manual. dumpsys to check current TextClassifierService. Change flag
to another service, and then check TextClassifierService changing to
expected TextClassifierService.
Test: Manual. Use local TextClassifierService to make sure the service
call order of connection callback and API call is correct.
Change-Id: I37ff298f0c65ae0469bd54e30f1ebf48b77c126c