Add an extra to HOME intent to indicate it comes form recents.

This allows the launcher to know how to handle HOME press properly
when a video is playing in the background. Without this, the Launcher
dismisses it's UI.

Launcher CL: https://googleplex-android-review.git.corp.google.com/#/c/1043435/

BUG: 28173127
Change-Id: Iab0c5013fc41f79a6fd45f49904edd0d0f9c17ff