Support double-tap home to go to recents.

Add config resources to enable the following behavior for
certain devices:

* press home - go home
* long-press home - launch assist intent
* double-tap home - open recents

Bug: 8891502
Change-Id: I4564d9e4d127f782ed5bd8eb613ef8f2255176cc
(cherry picked from commit b7805698035622a633b2d76f7ca60169aaa9f0ca)
Conflicts:
	policy/src/com/android/internal/policy/impl/PhoneWindowManager.java