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
DocumentsUI: Allow open files instead of URIs

Check files' real path and mime type to open them only in standalone mode.
This allows, for example, installing apks.

Change-Id: I4501c694db84b8595cd3928752254a8914539972
services: don't clear wallpaper when SystemUI
disconnects.

When we restart SystemUI (or the user), sometimes
the wallpaper would get cleared. This patch just
captures its originating component and
if that is our SystemUI component, ignore the event.

Change-Id: I3e159dc2c013e7d7e552e3c0cef55e3ab970ca87
Signed-off-by: Roman Birg <romanbirg@gmail.com>