Adds method to update the status of the Empty View.

The updateEmptyStatus() method checks if there is data to display or not, if there is data it hides the empty view, otherwise it will show it. That method is taken directly from the Android source code (https://github.com/android/platform_frameworks_base/blob/master/core/java/android/widget/AdapterView.java#L730-L754).
Then the method is called when the AdapterDataSetObserver gets invalidated (onInvalidated) or the data on it changes (onChanged) before requesting a Layout pass.
Adds example to show that the change with the Empty View works.
Adds onLayout method call after updating the Empty View status.

It seems to work without it but reading the comments above it, it seems to handle an edge case. See: https://github.com/android/platform_frameworks_base/blob/master/core/java/android/widget/AdapterView.java#L747-L749
Undoes changes in imports in SampleAdapter (AndroidStudio's fault)

Also adds blank lines and empty line at end of file in activity_sgv_empty_view.xml.
Merge pull request #33 from xrigau/master

Merge pull request #33 - Adds support for empty view in ExtendableListView