Add scrim below TaskMenu

When opening a menu, a scrim with 80% alpha should be shown on top of
Recents view.

Align task menu second row with icon: when the menu shows up in the
bottom row in landscape, the menu should be aligned on the second row.

TODO: there is a RTL bug that I'm waiting because it also affects other
parts, not only this menu.

Bug: 193432925
Test: open Overview and tap the app icon
Change-Id: I6846ee937cb5e739e8be64d17045bc3b32e28e46