Map mediaprovider to priv_app in 26.0 compat mapping

Commit: bde5c801 added a new type,
mediaprovider, which is being applied to an object (process) formerly
labeled as priv_app. Add the new type to the versioned attribute for
priv_app so that any vendor policy written for interaction with
mediaprovider continues to work.

Bug: 62573845
Test: None.  Prebuilt-only change.
Change-Id: Id98293369401a2af23c2328a1cb4a5bb2258aac8