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
(cherry picked from commit 8ccd106b2e09040ca3bc2fef9d231368d7dfe3dd)
Fix Icon size in DocumentsUI

For XXHDPI devices if you go into Settings/Display and Notification light and select a custom value (application) you will see a GIANT icon.

Before - http://i.imgur.com/DRkua3T.png

After - http://i.imgur.com/f7xW4tY.png

Change-Id: Ia355bddc515906134c8b3b4e67d381aaacceec78
(cherry picked from commit e4b838559599854c624d019c92a55a824dabf3ea)
hide search assistant arrow from lockscreen when custom lockscreen is enabled

Change-Id: I02d8286dd9f07cdb1de8a61101e772be35f65fda

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java

(cherry picked from commit 0fe2fa43607d575437092db37b13df8d29b1c350)