[Tizen] gbs-build crosswalk8 rpm fail.

Exclude unnecessary directory android_tools when build for tizen.
The size of src/third_party/android_tools directory in Crosswalk8 is almost  5.6G.
After add  --exclude=src/third_party/android_tools to src/xwalk/packaging/gbp-flat-tree.sh.
The rpm can build success.

BUG=XWALK-2224