Only wait for mGivenInsetsPending if providedInsets is null

If an insets source window uses any InsetsFrameProvider to modify the
insets, the givenContentInsets or the givenVisibleInsets won't be
applied. There is no reason to wait for mGivenInsetsPending before
updating the insets source or the insets hint.

Fix: 308874978
Test: 1. Open a landscape immersive app.
      2. Enter split-screen with another app in landscape mode.
      3. Double click on the divider to swap apps.
      4. Drag the divider all the way to the screen edge to maximize the
         immersive app.
      5. Make sure navigation bar is invisible.
Change-Id: I220f1b29375f38096b7df39fca0c4a93f527d1ee