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 89252ce3)

Change-Id: I44e01bee50125f4e6e5fff6e74c53bb2022ce355