[WIP] Multi-window

This is a big work in progress, which currently has some bugs,
but begins to look okay.

You can already multitask a little bit, by using the
app switcher and long-pressing an icon, then select "Add to
Split View".
Last window touched gets the key focus.

Code is very ugly, I know. Don't merge it, don't use it
for now. Unless you want to make your code ugly.

Enjoy :)

Change-Id: I4a03a84539fd3ac1273f0d3503fe8fafcd16be26
Avoid layout passes when updating data activity indicators.

ImageView can optimize out layout passes if old and new drawable have
equal sizes. Take advantage of that by displaying a transparent drawable
with the same size as the in/out drawables when there's no activity
instead of clearing out the drawable in that case.

Change-Id: Iaaa556ffab3ed4be1da8016ed40d0a9c2efcde01
Make ImageView drawable update more efficient in some cases.

When updating the view's drawable via setImageResource or setImageURI,
make sure to skip the layout pass if the old and new drawable have equal
sizes. This follows what is already done for setDrawable().

Change-Id: If73d225e38d0a2f7c616c4aeb33f67bdaed390f1
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
Add a circlular memory indicator to the recent panel view (1/2)

Originally picked from Chameleon OS
http://goo.gl/vgL71x

Modifications:
It allows to be toggled on/off as well as setting a custom location.

Colors are not solid, they have an alpha channel to provide
transparency, this way it doesnt interfer much with the recents
panel view.

Screenshot:
http://goo.gl/Th7g0j

OmniGears Part:
https://gerrit.omnirom.org/#/c/3057/

Change-Id: I07999d476a16d24ddf9c6de441ae646887da8074