DocumentsUI tablet support.

Shows as a faux dialog on larger tablets, since the normal dialog
themes from framework don't support action bars.  Instead, this hacks
itself to look like a dialog with an InsetDrawable.  Detects touches
in dim area to dismiss dialog.

Show all action bar icons on tablets, and roots panel is always
visible with a different shadow. Show settings in dialog. Allow mode
switches in recents.

Bug: 10329832
Change-Id: Id02982ce7b0d4694962d32f8d5286fdfcc207208