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: https://code.google.com/p/android/issues/detail?id=211420
Change-Id: Ic53da981ac0fa45bfed62e7b351d75dca0540235