emulator: Use distinct serial names for simultaneous qemu pipes.

ADB local transport for the emulator based on qemu pipes uses a socket
transport. Before this CL, multiple connection requests accepted by the
qemu pipe device would result in ADB transport with the same serial.
The register function would fail as a result, and all subsequent
connections would fail.

Test:
while true; do adb kill-server; adb devices; done
Doesn't fail for > 10 minutes.

This CL replaces an earlier CL
(I0fdcf2694516151c5f8f8e1580648b940679c981) that was unsafe for real
devices.

BUG=27441661

Change-Id: I7d801b175f3bee10fc7e0ab1b12d5623984371b9