Take another pass at detecting USB connections.

For some odd reason, devices with ADB connections in test harnesses
are reporting that they're connected over an AC charger instead of
USB charger.

So switch to reading a sysfs file to detect an active USB connection.

Test: builds, boots, rescue disabled when USB connection present
Bug: 34872406
Change-Id: I4c2cb9ed7e51b9692ce81eb0ef4945dae0036212