[Android] Cancel library-not-found dialog when library installed

When app startup, it will try load runtime lib. If failed, an
alert dialog will be prompted to navigate user to plat store.
When app's activity back on screen, it will check runtime lib's
existense.
If runtime lib found, should cancel the alert dialog automatically.

BUG=https://github.com/crosswalk-project/crosswalk/issues/638