Do not clear Task#mMultiWindowRestoreWindowingMode on exit split transitions

Task#mMultiWindowRestoreWindowingMode should be cleared whenever the
windowing mode changes as dictated by Activity#requestFullscreenMode.
However, when fullscreen is requested by an activity in split, the
windowing mode is changed throughout the transition before the windowing
mode is set to WINDOWING_MODE_UNDEFINED so that it inherits its parent's
fullscreen windowing mode.

This cleares the saved restore windowing mode which causes the request
fullscreen API to stop working.

In the future, we want to fix the API so that cases like this where the
windowing mode did not really change does not cause the saved windowing
mode to be clear, but to fix this by 25Q2, we (1) to not clear restore
mode if windowing mode did not change and (2) set windowing mode to
WINDOWING_MODE_FULLSCREEN instead at the end of the split transition.

Bug: 402973271
Flag: com.android.window.flags.enable_request_fullscreen_bugfix
Test: enter/exit fullscreen using API
Change-Id: Ibb7d9d47a38321b90170340498b31ee9a57707be