[Android] Fixes #979 - Changes in test pages won't rebuild XWalkCoreTest

With testing, if the files in 'sources' get updated (touch or content
change), the 'none' type target won't get rebuilt.

Instead, 'variable': {'additional_input_paths'} works as wish, that how
this commit comes.

BUG=#979
Merge pull request #991 from halton/android_test_gyp

[Android] Fixes #979 - Changes in test pages won't rebuild XWalkCoreTest