Camera2: On stopping preview, wait until captures are completed.

If preview is stopped while a still capture or a video snapshot is
underway, wait until the capture completes before stopping preview.

Also use the same behavior for disconnect - do not shut down the
camera device until captures are complete.

This works around HAL implementations which report to be done
(get_in_progress_count returns 0) even when a capture is still
outstanding.

Bug: 7276954

Change-Id: I66290acf1740cc330eadecbcded0c59fa9c5d2fd