Allow adbd to write to /data/adb

adbd writes debugging information to /data/adb
when persist.adb.trace_mask is set. Allow it.

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

(cherry picked from commit 973877db)

Change-Id: Ida2e0257c97941ab33ccdab59eb2cde95dca344f