recovery: Only install adb keys once

 * Recovery is currently calling setup_adbd when the process starts to
   fetch the keys from the data partition and (re)start adbd.
 * This of course has the heinous side effect of making it
   impossible to use gdbserver to debug recovery, making me a sad panda.
 * Make setup_adbd a pseudo-binary, and run it as a oneshot service
   instead.

Change-Id: Ib4b9e342564bb60bdc2df4cee91566f31c7db8f1