[Android] Rewrite generate_xwalk_core_library_aar.py.

The previous version was horribly bloated and broken. The main
issue (which is blocking XWALK-1930) is that CopyBinaries() recurses
into some directories and tries to copy each subpath individually. Not
only is this entirely unnecessary, but the code does it the wrong way
and, for ARM builds, creates something like
'jni/xwalk_core_library_java.jar/armeabi-v7a/libxwalkcore.so'.

Instead of just fixing one issue, rewrite the whole script in a saner,
much smaller and more Pythonic way:

* Remove a lot of unnecessary functions.
* Get rid of the "-s" option which was not being used at all.
* Stop calling sys.exit() in main() since it's supposed to return an
  exit code instead.
* Stop copying files from xwalk_core_library/ into an intermediate
  directory and just write them to the zip file directly.

(cherry picked from commit df68e4f379650d503d581bcbb5a6faf25fdf30e5)
[Android] Remove xwalk_core_library_java.jar from the AAR package.

This fixes another regression inherited from the previous version of the
script, and is also required for XWALK-1930: xwalk_core_library_java.jar
is not supposed to be packaged twice, it should be present as
classes.jar and not be part of jni/.

(cherry picked from commit c1775946066d19924eba5b66d0a7a59dbdb94828)
Merge pull request #2429 from rakuco/backport-aar-fixes

[Android] Backport fixes to generate_xwalk_core_library_aar.py.