WindowManagerService: Fix application transitions in immersive mode.

Change-Id: If7d3fc3ab566dedc3a5b38cd0a6dc3185f494e20
Signed-off-by: Carlo Savignano <stevewatersy@gmail.com>
DocumentsUI: Add a standalone File Manager

This commit adds a standalone mode to the DocumentsUI, to let
it act as a file manager/explorer.
You can browse your storage contents, open files, and delete them,
as well as do everything that the DocumentsUI can do (ie. sort
by name/size/type, list and grid view, recents, search, ...).

This gives us a consistent file browsing UX accross all apps
that uses the new documents API.

Change-Id: If73a3c100f010bdb766c843976d4fd3573ea805c
core: update to new settings icon