Only bring desktop app to front when necessary

If the active desktops apps are already in the front, then there's
no need to add WCT#reorder operations for every desktop app.
Instead, check first whether there is an invisble app, and if there
is, reorder all apps so that the Z order is maintained.

Bug: 255344915
Bug: 258056163
Test: atest DesktopModeControllerTest
Test: manual - in desktop mode,
 1. Launch Gmail
 2. Launch Photos
 3. Observe there is no reorder triggered anymore
Change-Id: Ib2cc8d900d1d0ccc8b9e2b49b95f1708531aa398