adb: fix host-side serial number parsing for IPv6.

When the adb client sends a command to the adb server targeting a
particular device serial, it looks something like this:
  host-serial:<serial>:<command>

But if <serial> happens to be an IPv6 address (e.g. when `adb connect`
targets IPv6), the current parsing code doesn't handle the additional
colons properly. This CL fixes the host-serial parsing to handle this
case.

This only affects commands that explicitly name a device serial, e.g.:
  adb -s <IPv6> shell
  adb -s <IPv6> forward <port> <port>
Implicitly using a single attached device was unaffected by this bug.

Bug: http://b/30891386
Test: `adb -s [fe80::ba27:ebff:feb1:934%eth2]:5555 shell` works now,
      and new unittests pass.
Change-Id: Iffe784e61432ae94eb96ed3c8477900a3e807329