Provide a stub implementation of device::UsbService on Android.

By stubbing out the Android implementation of UsbService we can avoid
the #ifdefs necessary to disable WebUSB on Android since it will be
implemented eventually anyways.

BUG=549257

Review URL: https://codereview.chromium.org/1450733002

Cr-Commit-Position: refs/heads/master@{#360346}
15 files changed