adb: disable ReconnectHandler in adbd.

Previously, when a TCP connection was disconnected from adbd, we were
registering it with ReconnectHandler, which led to the transport
sticking around after the socket was closed. Due to the naming of
TCP transports in adbd (host-<fd number>), this results in incoming
connections being immediately closed if their file descriptor number
ends up being the same as a TCP transport that had previously
disconnected.

Guard all of the reconnect logic with ADB_HOST, to fix this.

Bug: http://b/112054041
Test: while true; do adb connect <device>; adb connect <device>; adb shell true; done
Change-Id: Ib55d304d7e07d6d744e8321d34671bb6d4b91afe