Allow multiple (sequential) adb users on Linux.

Before this, adb will fail to start for the second user who tries because
/tmp/adb.log already exists and isn't writable by the second user.

Also allow $TMPDIR to override the use of /tmp.

Bug: http://b/29201686
Bug: https://code.google.com/p/android/issues/detail?id=211420

(cherry picked from commit d89a6c22)

Change-Id: Ibb1369e8043feb9774593e317dcd7bb93323fd89