DocumentsUI: Stop to override fixed size of dialog in CreateDirectoryDialog.

Previously CreateDirectoryDialog overrides fixed size defined in the application
theme. As a result, the dialog is shown as vertically long.

The CL specifies explicitly specifies the default theme for dialog when creating
the dialog to stop override the application theme.

BUG=20239384
Change-Id: I7227bd91705c32f5196b666bbe94ff32b5cb4af8