Introduce FakeMojoUsbDeviceManager and use it in a unit test suite.

As device::usb::DeviceManagerImpl will be moved to the Device Service,
it should only be visible in //service directory. For unit tests out of
//service, this CL introduced FakeMojoUsbDeviceManager and used it in
UsbChooserContext unit test.

Bug: 699790
Change-Id: Ifb8bc80b5c5e44d25a4862630dd8d746572c12d2
Reviewed-on: https://chromium-review.googlesource.com/c/1238355
Commit-Queue: Donna Wu <donna.wu@intel.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598257}
22 files changed