[adb] Disable _adb._tcp. service auto-connect by default.

Added ADB_MDNS_AUTO_CONNECT envionment variable to control which
services to allow auto-connect. By default, only _adb-tls-connect
services can auto-connect, since these services only auto-connect once
paired. _adb services will try to auto-connect to every service found.

Bug: 152636135

Test: ADB_TRACE=1 adb server nodaemon | grep whitelist
Test: ADB_TRACE=1 ADB_MDNS_AUTO_CONNECT=adb adb server nodaemon | grep whitelist
Test: ADB_TRACE=1 ADB_MDNS_AUTO_CONNECT=adb,adb-tls-connect adb server nodaemon | grep whitelist
Test: ADB_TRACE=1 ADB_MDNS_AUTO_CONNECT=adb-tls-connect adb server nodaemon | grep whitelist
Test: ADB_TRACE=1 ADB_MDNS_AUTO_CONNECT=0 adb server nodaemon | grep whitelist
Test: ADB_TRACE=1 ADB_MDNS_AUTO_CONNECT=1 adb server nodaemon | grep whitelist

Change-Id: Ie562ea24fea3d6d96e67b376a0523b09e2778eb7