fuzzy_fastboot: use ParseNetworkSerial for parsing tcp device serials

The current way of parsing tcp device serials doesn't allow for
specifying port numbers - the port gets hardcoded to 5554.
Use ParseNetworkSerial for converting the device serial into an address
and a port (the default value is still 5554).

Bug: 372958321
Test: fuzzy_fastboot --serial=tcp:DUT:5554
Change-Id: Ie86ea6e188a41e450a98ff90d1fdb47b2a30f340