Clear "restore" mode when setting windowing mode

Whenever Task#setWindowingMode is called,
mMultiwindowRestoreWindowingMode is cleared.
Activity#requestMultiwindowFullscreen will now set the task's
mMultiwindowRestoreWindowingMode after setting the new windowing mode
when requesting to enter freefrom. That way, the request fullscreen api
can only restore the windowing mode if no windowing mode change occurs
between the time a task requests to enter and exit fullscreen.

Bug: 331649648
Test: Enter fullscreen using api, exit and re-enter fullscreen using the
caption handle, attemt to exit fullscreen using the api. App should stay
in fullscreen.

Change-Id: Ia379d8dc4b9a57352a35b0504815a88f1c9fdbd6