adb: libusb: replace sleep with timed CV wait.

Instead of sleeping for 500ms at the end of every device poll loop, use
a timed condition variable wait so that we can tell the device poll
thread to immediately commit suicide.

Bug: http://b/37869663
Test: adb kill-server; adb start-server
Change-Id: I597071866f7d9ef91900411727345d32c1a97556