adb: libusb: wait for devices to become accessible.

Android's host linux libusb uses netlink instead of udev for device
hotplug notification, which means we can get hotplug notifications
before udev has updated ownership/perms on the device.

When detecting a new device, poll the device file for a while until we
can access it, before trying to open it.

Bug: http://b/38170349
Test: manually incrased timeout and chmodded a device betwen 0 and 664
Change-Id: I3c714f630940df02b407442592301e2bbb3d9653