Users can implement focus movement by SetCustomAlgorithm.

Users can implement focus movement by SetCustomAlgorithm.

So User implemented focus movement by SetCustomAlgorithm(AutoFocusAlgorithm.Instance);

And if user do SetCustomAlgorithm(null), the user expects the focus not to move.

this is for backward-compatibility.

Add EnableDefaultAlgorithm() and IsDefaultAlgorithmEnabled() for enable/disable the default algorithm

Change-Id: I231508ace0f378edc268731e80ac7878af5cd650