[Android] Remove duplicated asset files for Android tests.

There are 3 instrumentation tests for different purposes. They use
some common files. However, currently these files are copied in
many places. This fix is to share all of these asset files in a
shared folder: test/android/data. If a test APK needs them, copy
them in the generated output folder.

This is the first step: share asset files. Next step is to share
Java code.

BUG=https://crosswalk-project.org/jira/browse/XWALK-95