[Android] Export xwalk core library as a jar

Previously, xwalk core library is containing bunches
of java source code, even including many generated
code. It's hard to maintain after chromium rebase.

With this commit, merge_jars.py will help merge all the
jars from chromium and xwalk component needed by xwalk
core library into one xwalk_core_library_java.jar.

Only two things missing:
1. The R.java for resource mapping still needs to be copied
separately, because they are not included in any jar.
2. As well as the NativeLibraries.java.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1234