Minor fixes on ContentCapture session logic.

Some apps - like launcher and settings' FallbackHome - generate view-level events before ever
starting an activity (probably because they're using dialogs). Hence, we were scheduling a
flush request, but never cancelling it.

Bug: 120494182
Bug: 122959591
Test: atest CtsContentCaptureServiceTestCases

Change-Id: I4a5448f19902ae51c8f2679eb0c468757b98c3ff