fuzzy_fastboot: use 'tcp:' prefix to identify fastboot protocol.

The fastbootd over ethernet was implemented with IPv6 link-local
address. An IPv6 address may include many ':'. It will break this fuzzy
test. This new solution uses "tcp:" prefix to identify the fastboot
protocol, like the host fastboot command.

BUG: 157887327
Test: fuzzy_fastboot --serial=tcp:fe80::230:1bff:feba:8128%wlan0 \
      --gtest_filter=*Logical*
Bug: http://b/166279510

Change-Id: I8fe7e6e3ade94a26e05a31ac20ed9ab3839dd342
Merged-In: I971fd9e25741e18bf7f5907d562556b09db1d624