Add explicit close of Scanner input

The UsbDeviceManager is using a Scanner to identify
the ALSA card/device numbers to use for USB Gadget
audio, with a FileInputStream as its input.

Since FileInputStream implements the Closeable interface
it provides a close() method that should be explictly
called in order to close the stream source and release
any system resources that it might hold.

Change-Id: Ia0ff86d1f3cdf8916becec9c8603915dcbf4d2c8