Changed how the Smart Suggestions service is defined.

Initially it was defined as secure Settings, but the proper way is to let the
OEMs set it by overlaying a framework resources, which an option to temporarily
override that setting using a Shell cmd (so it can be changed on CTS tests).

This CL also changes how the service can be explicitly enabled / disabled at
the time the system starts, so it can be disabled by a server-push in case of
emergencies (and also manually enable by developers when running the CTS tests
on AOSP builds).

Bug: 119776618
Bug: 111276913
Bug: 117779333

Test: adb shell settings put global \
   smart_suggestions_service_explicitly_enabled true && \
   sleep 10s && \ adb shell stop && adb shell start \
   atest CtsContentCaptureServiceTestCases
Test: atest FrameworksCoreTests:SettingsBackupTest
Test: atest CtsAutoFillServiceTestCases # to make sure it didn't break
Test: m -j SettingsProvider

Change-Id: Ibd1bba0c0d534b4b6344516d8ff893142785a57a