combine adbd.rc with init.usb.rc

init.usb.rc and adbd.rc contain similar contents and belong in the same
file.

This file also belongs on the ramdisk as adbd is on the ramdisk, not the
system partition, therefore resolving to keep init.usb.rc in its current
location and combining the contents of adbd.rc is the best approach

Change-Id: I430f8fea58694679e7b8b7be69ce87daadd616f4