判断摄像头是否存在为什么使用的是PackageManager.FEATURE_CAMERA 而不是PackageManager.FEATURE_CAMERA_ANY
FEATURE_CAMERA: The device has a camera facing away from the screen(这台设备有一个摄像头，对着屏幕)
FEATURE_CAMERA_ANY: The device has at least one camera pointing in some direction, or can support an external camera being connected to it(设备至少有一个指向某个方向的摄像机，或者可以支持连接到它的外部摄像机)
判断摄像头是否存在为什么使用的是PackageManager.FEATURE_CAMERA 而不是PackageManager.FEATURE_CAMERA_ANY FEATURE_CAMERA: The device has a camera facing away from the screen(这台设备有一个摄像头，对着屏幕) FEATURE_CAMERA_ANY: The device has at least one camera pointing in some direction, or can support an external camera being connected to it(设备至少有一个指向某个方向的摄像机，或者可以支持连接到它的外部摄像机)