desktop-exploded-view: Add header to individual windows.

The header has an app icon, app title and a close button. Currently
clicking on the close button does nothing. Will address it in a
following CL.

Flag: com.android.launcher3.enable_desktop_exploded_view
Test: Manual
Bug: 353965691

Change-Id: I5e0303a02d5f8afd16a5f5f0335a8fa489ff98f2