init: remove adb forced config and switch to mtp.

-adb is not enabled by default and now requires a whitelist, for security
purposes.  Let's enable MTP in that case (this will alleviate users
having to enable it manually) and leave the debugging option up
to developer settings.

-Note: the forced adb config was there to allow for adb to start
       without delay on system init, for early debugging purposes.