adb: libusb: switch to hotplug for device detection.

Switch from polling in a loop to using libusb's hotplug API to detect
when devices arrive and leave. Use this to remove devices that were
inaccessible when they're unplugged.

Bug: http://b/38170349
Test: plugged in device
Change-Id: Id157412eb46834debecb0cd45b47b1ced50c2274