Handle user stopping in AppSearch.

When a user stops, we can close its AppSearchImpl instance to release
resources. We'll also remove it from our cache of unlocked users so
future API calls won't execute on a locked user.

Bug: 179407490
Test: atest --rebuild-module-info CtsAppSearchHostTestCases
Change-Id: I09953d32ad4aaf2989386e99c93623b58d564c27