libvideoeditor: fix cannot show some thumbnails in storyboard or no frame...

libvideoeditor: fix cannot show some thumbnails in storyboard or no frame display in PreviewRenderer surface after importing video

1. this is caused mFrameIntervalMs isn't assigned for use-software-codec case
2. without the assingment, some decoded frames will not be copied to the buffer queue of VideoEditorVideoDecoder

Change-Id: I578954e38958ebac8f258300e60b8ddf60e50521
Signed-off-by: beanz <beanz@marvell.com>