Ensure /data/adb exists

Ensure that /data/adb always exists. This directory is used
for writing adb debugging information when persist.adb.trace_mask
is set.

Bug: https://code.google.com/p/android/issues/detail?id=72895

(cherry picked from commit 89252ce31aacb27248e708b6487f730d3a024943)

Change-Id: I44e01bee50125f4e6e5fff6e74c53bb2022ce355
diff --git a/rootdir/init.rc b/rootdir/init.rc
index 8ba7c10..9093b54 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -309,6 +309,8 @@
     # the following directory.
     mkdir /data/mediadrm 0770 mediadrm mediadrm
 
+    mkdir /data/adb 0700 root root
+
     # symlink to bugreport storage location
     symlink /data/data/com.android.shell/files/bugreports /data/bugreports