fix return behavior of deleteKeyphraseSoundModel

When deleting an enrolled keyphrase sound model that is not actively
started, the model is never removed from the framework database, and an
incorrect value is returned.

Test: gts-tradefed run gts -m GtsAssistIntentTestCases \
-t com.google.android.assist.gts.KeyphraseModelManagerTest \
\#testUpdateAndRemoveKeyphraseSoundModel
Bug: 169628490

Change-Id: I469fe1fd05e386095ec646e1506fcb191b93c46f