Gallery layout is triggered when parent element is hidden (width=0)

[cvolzke4]

A re-layout of the photos happens when width is 0, ie. when the containing element is hidden.
This causes the following problems:

1. For an app with tabs, the scroll position of the gallery is not maintained when changing tabs.
2. A complete re-layout is performed, which causes images to re-render causing flicker.

Screencast before:
share.getcloudapp.com/2NuybPlb

Screencast with a fix:
share.getcloudapp.com/6quebvrB

[cvolzke4]

Feel free to ping me at cvolzke@google.com if you have any questions.

We'd like to use this in our Story Editor app.

PR: #184