Ensure dim layer not going away when changing activity

Given two activities A and B requesting dim, if B opens on top of activity A, we need to make sure that we don't fade out and in again the dim layer, as this causes useless animations. To do this, we need to ensure that during the closure of A and the opening of B, there is always a visible container with the dim,
which means that A will keep the dim until B is fully visible.

A practical example in which this was failing, was the transition
ChooserActivityLauncher -> InstallStaging -> PackageInstallerActivity
when installing an app from an on-device .apk

Test: DimmerTests
Bug: 295291019
Bug: 281632483
Change-Id: I18b1c9c8051147ca15b2e7087a8a48336cdf76ed