Move adbd authentication into its own library.

As part of moving adbd into a module, split authentication out into a
library with a stable ABI that lives on the system image.

Bug: http://b/137798163
Test: for i in `seq 1 100000`; do echo $i; adb wait-for-device shell "su 0 stop; su 0 start; sleep 10"; adb disconnect; done
Change-Id: I05e3679598e33c83f8cc4c17208f4f753cbfd5e1