Fix flaky testVisibleWithInsetsProvider test

The test would always fail if we wait until the handler idle. The reason
is, when layoutAndAssignWindowLayersIfNeeded is called, in
DisplayPolicy, the mTopFullscreenOpaqueWindowState will take the control
of the insets visibility back by calling onControlChanged. At that time,
the visibility will get back to true, no matter what happened before.

Mock the DC and let the function do nothing would help the test to be a
unit test and all the test will be no longer flaky.

Test: atest WindowStateTests
Bug: 74078662
Bug: 69229402

Change-Id: I91255e2e7d5c8f2dcdf3faf72aa87c4df77a94ec