adb: add option to disable kill-server.

In the post-apocalypse, it's increasingly common for people to use ssh
port forwarding to use an adb client with a remote adb server. This
results in `adb kill-server` being catastrophic, because the client has
no way of restarting the server on the other end. Android Studio in
particular has an unforunate habit of trying to manage adb's life cycle
such that starting or exiting Studio will result in a kill-server.

Add the ADB_REJECT_KILL_SERVER environment variable which ignores
kill-server, to make this scenario a bit better.

Bug: http://b/152251952
Test: ADB_REJECT_KILL_SERVER=1 adb start-server; adb kill-server
Change-Id: I5533a6dcbdb9220526a6fcf9ca31d9fcef1cec17