Refine excludeInvisibleImeFrames

When adding a window to WMS, its z-order may be higher than IME
initially. And then, WMS will move IME to the top of the new window.
Although IME is invisible during launching an app, that causes a change
in WindowState#mAboveInsetsState. And it would trigger a redundant
onApplyWindowInsets at the client side.

This CL treats null IME source as invisible IME source in
InsetsState#equals when excludeInvisibleImeFrames is true.

Bug: 237749017
Test: Add logs in dispatchApplyWindowInsets, and see if it gets printed
      twice while launching an app.
Change-Id: I5eba7732619f7ceebe3152e81037a43e9ab3d91f