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
[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 :)

PS 11-18: Rebase

Change-Id: I4a03a84539fd3ac1273f0d3503fe8fafcd16be26
Signed-off-by: zwliew <zhaoweiliew@gmail.com>

Conflicts:
	core/java/android/app/IActivityManager.java
	core/java/android/inputmethodservice/InputMethodService.java
	packages/SystemUI/res/values/custom_strings.xml
	packages/SystemUI/src/com/android/systemui/recent/RecentsPanelView.java
	services/java/com/android/server/am/ActivityStack.java
[3/3] core: Add Dialer suggestions

Change-Id: I01b27afb075e36d8d807d2fdbf5032ddb6e10641
Signed-off-by: zwliew <zhaoweiliew@gmail.com>

Conflicts:
	core/java/android/provider/Settings.java