SnapdragonCamera: fix screen flash on resuming for video

Currently preview cover will be hidden shortly after preview started,
but if the actual first frame comes late, the main screen will flash.
Register a oneshot preview callback to hide preview cover.
Register the callback before calling startPreview.

Change-Id: Ide0dd4b2eebe7cb73a2192f11c4ea3a471718c05
CRs-Fixed: 911138
1 file changed