Use the copy constructor to create simulated sources

In this way, we don't need to modify the code in createSimulatedSource
when there is a new field of InsetsSource.

This fixes a regression since 244e9600

Fix: 285080829
Test: Open Settings from portrait while holding device in landscape, and
      see if the nav bar area is transparent in gesture nav mode.
Change-Id: I8288f9d9bf6f2f28a71448cc35727c1c3955bad7