InsetsSourceProvider: Fix serverVisible update order in onPostLayout

Fixes an issue where serverVisible was only updated after updateFrames, but
updating the frames depends on the server visibility being correct since [1].

Also addresses an issue in the test fixture which prevented this regression
from being caught: the server visibility is now set by onPostLayout, instead
of onSetup, to mirror how the server visibility would be set at runtime.

[1]: Id2c482e112c1d73cf7d5b3ba5e1a2d5775f47298

Fixes: 156226586
Test: atest ScreenDecorWindowTests InsetsSourceProviderTest
Change-Id: Id90e6315e9fe230307929a8d992a76a5dae1107c