Move adbd to an apex.

Test: adb shell "su 0 readlink /proc/\`pidof adbd\`/exe"
Change-Id: I84dfe4d1b28b619f98c03a2c8eeef2c783d30af2
diff --git a/rootdir/init.usb.rc b/rootdir/init.usb.rc
index b6cba90..688d9ad 100644
--- a/rootdir/init.usb.rc
+++ b/rootdir/init.usb.rc
@@ -16,6 +16,7 @@
     class core
     socket adbd seqpacket 660 system system
     disabled
+    updatable
     seclabel u:r:adbd:s0
 
 on boot