Settings: ButtonSettings: Introduce recents long press action

Add a new preference in Settings > Buttons > Recents long press action
that controls the behavior of recents long press intent
ACTION_RECENTS_LONG_PRESS.

The user preference is saved in
Settings.Secure.RECENTS_LONG_PRESS_ACTIVITY; if empty the default is the
system's "switch to last app" behavior. Settings.Secure is used
intentionally over Settings.System for permission reasons.

Change-Id: Ifc61b5958a88360e32db31459f0205b6ba5306b3