[Android] Make prepare_r_java.py an executable

IDEs like eclipse are unable to run the program as an external builder
if they are not marked executable.

Also, use copy2 instead of copyfile to preserve 'x' bit in file mode.
[Android] Add eclipse build support for xwalk_core_library

Eclipse is a popular IDE for android development. So it's useful
to have this supported out of the box.

This change adds the .project, .classpath files required by Eclipse
for importing.
Merge pull request #992 from gramakri-intel/master

[Android] Eclipse build support for xwalk_core_library