ANDROID: GKI: enable CONFIG_USB_DUMMY_HCD

A number of systems need the dummy USB host controller driver for
testing, so enable it into the kernel to remove the need to support a
bunch of exported symbols just for that driver.

Bug: 157965270
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I74b3aa819984dd894cccd3a5239d210b9e0d43a5