Custom OssLicensesMenuActivity/component and/or full flexibility on display

**Problem**:
If the application theme in the manifest is, for example, a "no action bar theme", the title can't be displayed, since I can't override the theme for `OssLicensesMenuActivity`.
Please let us subclass the activity or at least set a theme on it.

A better option is **making the list a component (fragment?) that I can include in my own activity.**

The best solution, however, is to have also **a method that returns a data structure (list of licenses and dependency-license mapping)** so I can display it in my own way.
For example, I might want to group by license, i.e., list distinct licenses and, on each item's subtitle, list the packages using that same license.
This would also solve #73, since we would be able to manually add licenses not related to gradle or maven. For example, resources like icons, etc, which may require a notice somewhere, could be added in my custom implementation of the activity.
This is the best solution, since it gives us full flexibility on what info is shown and how.