Fix sidefps visibility.

In ag/21915461, there seems to be a regression where the side fps does
not hide when the bouncer hides. This is likely caused by a race
condition between the visibility and when startingToHide is called.

We can fix this by updating the side fps visibility when hide is called
instead of startingToHide.

Fixes: 273563764
Bug: 263924084
Test: added a unit test.
Test: Show bouncer with side fps and hide the bouncer.
Change-Id: Idacf9e74ed3c0f7c2a03d5dd6ffa128b49c50c05