Add ReceiverActivity to MtpDocumentsProvider.

To show the application chooser for the UsbManager.ACTION_USB_DEVICE_ATTACHED
intent, the intent should be received by activity. The activity has NoDisplay
theme and immediately terminate after routing intent to MtpDocumentsService.

BUG=20274999

Change-Id: I2080ea433b1657dd68640681664adbf92d1d30e9