[Android] File picker can't work on Android OS

This is a regression issue when chromium-crosswalk rebased to 34.
In Chromium 34, showCancelableIntent function is implemented in ActivityWindowAndroid.
When context is a Activity, we need a ActivityWindowAndroid instance instead of
WindowAndroid.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1086
Merge pull request #1691 from yinli/file_picker

[Android] File picker can't work on Android OS