Send a FLUSH event after each frame.

This `notifyContentCaptureEvents` is called after the view traversal. The CC events are buffered in the `AttachInfo.mContentCaptureEvents` during the traversal.

Send a FLUSH event at the end of the traversal to kick off the System Intelligence to process and rebuild the VH.

Bug: 380381249
Change-Id: Ie0dd7c2511ff584b3c46035693359ed9df69660e
Flag: android.view.contentcapture.flags.flush_after_each_frame
Test: atest FrameworksCoreTests:android.view.contentcapture.MainContentCaptureSessionTest