Don't increase notif importance when shouldSilence

If a notification already has a silent importance, the assistant
shouldn't suggest a higher importance.

Change-Id: I304743330a1183e3730b0d0b70b38d577f3dfad3
Fixes: 116810453
Test: Notification Attention Management Test (CTS Verifier)
Test: make ExtServices && adb install -r $OUT/system/priv-app/ExtServices/ExtServices.apk && atest ExtServicesUnitTests