add "adb sideload" and sideload connection state

Recovery will soon support a minimal implementation of adbd which will
do nothing but accept downloads from the "adb sideload" command and
install them.  This is the client side command (mostly resurrected out
of the old circa-2007 "adb recover" command) and the new connection
state.

Change-Id: I4f67b63f1b3b38d28c285d1278d46782679762a2