[Android] Compress AAR packages generated with Python's zipfile.

By default, Python's zipfile module will store the files in the ZIP
archives it creates uncompressed, which makes the architecture-specific
AAR archives we host in 01.org quite big. x86 AAR packages are more than
double the size of a compressed ZIP archive, for example (43M vs 20M).

Fix this by explicitly creating ZipFile with zipfile.ZIP_DEFLATED.

BUG=XWALK-2688
Merge pull request #2481 from rakuco/compress-python-generated-zip-files

[Android] Compress AAR packages generated with Python's zipfile.