adb: On Mac, clear both host and device endpoints at startup

This CL clears both the host and device endpoints right at the
beginning when the bulk endpoints are identified. This is in general
a "good idea", but more specifically for us, it fixes the issue
that sometimes when adb quits, it clears the endpoint on the host,
but not on the device which resulted in a subsequent invocation of
adb was seeing a stall.

Bug: https://code.google.com/p/android/issues/detail?id=182151

Change-Id: I331fa6805c40d1f50c153c010ceecd2f6a4045eb