Force pre-N apps to be resizeable.

We feel this experience is better than the 2-finger gesture.
Added ActivityInfo.RESIZE_MODE_FORCE_RESIZEABLE resizeMode
to indicate we are force resizing an app.

Bug: 27132829
Bug: 26847884
Change-Id: I65db2de0d9f3f171cc3bb136cc1282b3ef3549b0