adb: Fix adb tcpip command

When running "adb tcpip 5555", adb create a service socket named
"tcpip:5555". Only compare the 6 first chars "tcpip:" to decide if
we enable exit_on_close.

Bug: 6650130
Change-Id: I0835973de044f1cfde0e859ff0277c0ccc2630a3