Changed ContentCapture session lifecycle so session is kept alive when service died.

This is useful for long-lived activities (like a browser), so child session can still be reported
after the service is restarted after dying.

Test: manual verification (cannot be tested using CTS because it would kill the test process)
Test: atest FrameworksCoreTests:android.view.contentcapture.ContentCaptureEventTest
Test: m update-api

Bug: 128466656

Change-Id: I9310263e897e929189d323d31853592a374dc6e0