Define roles for Gallery and Music apps.

The new storage work in Q will extend capabilities to the default
Gallery and default Music apps, and we'll be implementing the
management of those default apps using the new roles feature.

This change starts by defining these roles using a relaxed definition
that only uses the existing CATEGORY_APP_* intent filters.

Bug: 119713234
Test: manual
Change-Id: Id6795cd9a8311e87b1b0b0a9482b6b322204233c