[Android] Refine the xwalk_core_test_apk target

This fix is to refine the test target to make it more cleaner to add
HTML testing file as apk asset file. Right now, when adding a new HTML
file for testing, you just need to update the 'sources' list without
adding a new entry item in 'copies' list.
Merge pull request #931 from hmin/refine-test-target

[Android] Refine the xwalk_core_test_apk target