unspecified(預設值，由系統判斷狀態自動切換),The default value. The system chooses the
orientation. The policy it uses, and therefore the choices made in specific
contexts, may differ from device to device.
user(用戶當前設置的orientation值),The user's current preferred
behind(下一個要顯示的Activity的orientation值),The same orientation as the
activity that's immediately beneath it in the activity stack.
orientation determined by a physical orientation sensor. The orientation of
the display depends on how the user is holding the device; it changes when
the user rotates the device.
nosensor(不使用感測器，這個效果差不多等於unspecified).An orientation determined without
reference to a physical orientation sensor. The sensor is ignored, so the
display will not rotate based on how the user moves the device. Except for
this distinction, the system chooses the orientation using the same policy as
for the "unspecified" setting.