Fix leaking the first activity while preserving windows.

This is achieved by not having the decor view hold onto the activity
context. Instead, we are wrapping application context, so that we can
have theme support and also have a special instance of window manager
that is aware of the phone window (the same way as activity do).

This reverts commit a5ffea3b.

Change-Id: I924f4c7ef8f0d20e9174bd7b3e00ec00b44443b9