Rename FOCUS_MODE_CONTINUOUS to FOCUS_MODE_CONTINUOUS_VIDEO.

This constant is not public yet. Continuous autofocus should
behave differently in still camera and camcorder. In camcorder,
lens movement may be more smooth. And the triggers to start a
new focus search may be different. If there is a need,
FOCUS_MODE_CONTINUOUS_PHOTO can be added in the future.

Change-Id: I05df9e491aca37829be3df92a73b952f26c86a4a