这个是要硬件支持的，不是所有camera都可以。
先调用getMaxNumFocusAreas()看下是否支持，google的说明：
Before using this API or {@link #setFocusAreas(List)}, apps should
call {@link #getMaxNumFocusAreas()} to know the maximum number of
focus areas first. If the value is 0, focus area is not supported.
